[1/2,v7] core: don't build host-cmake if it is available on the build host

From: Luca Ceresoli <luca@lucaceresoli.net>

Currently all cmake packages depend on host-cmake. Unfortunately
host-cmake takes a long time to configure and build: almost 7 minutes
on a dual-core i5 with SSD. The time does not change even with ccache
enabled.

Indeed, building host-cmake is avoidable if it is already installed on
the build host: CMake is supposed to be quite portable, and the only
patch in Buildroot for the CMake package seems to only affect
target-cmake.

Thus we automatically skip building host-cmake and use the one on the
system if:
 - cmake is available on the system and
 - it is recent enough.

First, we leverage the existing infrastructure in
support/dependencies/dependencies.mk to find out whether there's a
suitable cmake executable on the system. Its path can be passed in the
BR2_CMAKE environment variable, otherwise it defaults to "cmake". If
it is enabled, found and suitable then we set BR2_CMAKE_HOST_DEPENDENCY
to empty; otherwise we set BR2_CMAKE_HOST_DEPENDENCY to 'host-cmake' and
override BR2_CMAKE with "$(HOST_DIR)/usr/bin/cmake" to revert to using
our own cmake (the old behaviour).

Then in pkg-cmake.mk we replace the hard-coded dependency on host-cmake
to using the BR2_CMAKE_HOST_DEPENDENCY variable, and we use $(BR2_CMAKE)
instead of $(HOST_DIR)/usr/bin/cmake.

Unlike what we do for host-tar and host-xzcat, for host-cmake we do
not add host-cmake to DEPENDENCIES_HOST_PREREQ. If we did, host-cmake
would be a dependency for _any_ package when it's not installed on the
host, even when no cmake package is selected.

Cmake versions older than 3.0 are affected by the bug described and
fixed in Buildroot in ef2c1970e4bf ("cmake: add patch to fix Qt mkspecs
detection"). The bug was fixed in upstream CMake in version 3.0 [0].

Amongst all the cmake packages currently in Buildroot, the currently
highest version mentioned in cmake_minimum_required() is 3.1 (grantlee
and opencv3).

Thus we use 3.1 as the lowest required cmake for now, until a package is
bumped, or a new package added, with a higher required version.

[0] https://cmake.org/gitweb?p=cmake.git;h=e8b8b37ef6fef094940d3384df5a1d421b9fa568

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Davide Viti <zinosat@tiscali.it>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
[yann.morin.1998@free.fr:
  - simplify logic in check-host-cmake.mk;
  - set and use BR2_CMAKE_HOST_DEPENDENCY, drop USE_SYSTEM_CMAKE;
  - bump to cmake 3.1 for grantlee and opencv;
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

---
Results (by Luca, before Yann's changes):

On my build server, this patch reduced the build time for a batch of
16 Buildroot configurations (all building cmake-based packages) from
2h44m to 1h54m. Woah, it's a 30% saving!

Tested on:
 - Ubuntu 14.04 without CMake, with official CMake (2.8), PPA CMake
   (3.2)
 - Ubuntu 15.10 without CMake, with official CMake (3.2)
 - Ubuntu 16.04 without CMake, with official CMake (3.5)

Results (by Yann):

On my machine, this gains about 5min 30s per build, which is far from
negligible.

Tested, by artificially bumping/lowering the minimum required version,
on:
 - Ubuntu 16.04 without CMake, with official CMake (3.5)

---
Changes v6 -> v7:
  - drop the post-patch hook, moved to a follow-up patch  (Arnout)
  - eye-candy in script  (Arnout)

Changes v5 -> v6:
  - rename variables in the version check in pkg-cmake  (Luca)
  - fix commit log  (Luca)

Changes v4 -> v5:
 - adopted by Yann;
 - drop USE_SYSTEM_CMAKE in favour of BR2_CMAKE_HOST_DEPENDENCY which
   directly contains 'host-cmake' if it is needed;
 - unconditionally add $(BR2_CMAKE_HOST_DEPENDENCY) to dependencies: it
   is empty if host-cmake is not needed;
 - check our minimum version is enough in a post-patch hook.

Changes v3 -> v4:
 - rename USE_SYSTEM_HOST_CMAKE -> USE_SYSTEM_CMAKE (Arnout)
 - ditch BR2_TRY_SYSTEM_CMAKE and use system-cmake unconditionally
   if it is found and >= 3.0 (Arnout)
 - rename BR2_HOST_CMAKE -> BR2_CMAKE since it can be either a
   host-cmake (built by Buildroot) or a system-cmake

Changes v2 -> v3:
 - make this feature optional via the BR2_TRY_SYSTEM_CMAKE kconfig
   variable
 - rename the CMAKE variable to BR2_HOST_CMAKE, so it's coherent
   with the naming of other variables overridable by the
   environment, e.g. BR2_DL_DIR
 - invert the logic of the variable triggering the host-cmake
   dependency: USE_SYSTEM_CMAKE instead of BUILD_HOST_CMAKE;
   needed to implement BR2_TRY_SYSTEM_CMAKE

Changes v1 -> v2:
 - Require cmake >= 3.0. Fixes qjson failure (Luca, Samuel, Thomas)
 - In check-host-cmake.sh only search $1, not "cmake" (Arnout)
 - typo: host-ccache -> host-cmake (Arnout)
---
 package/pkg-cmake.mk                     |  6 +++---
 support/dependencies/check-host-cmake.mk | 18 ++++++++++++++++++
 support/dependencies/check-host-cmake.sh | 28 ++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+), 3 deletions(-)
 create mode 100644 support/dependencies/check-host-cmake.mk
 create mode 100755 support/dependencies/check-host-cmake.sh

On 11-09-16 17:58, Yann E. MORIN wrote:
[snip]
> +# Set this to either 3.0 or higher, depending on the highest minimum
> +# version required by any of the packages bundled in Buildroot. If a
> +# package is bumped or a new one added, and it requires a higher
> +# version, our cmake infra will catch it and whine.

 Without patch 2, this last sentence isn't true anymore. But let's assume that
patch 2 will be committed quickly :-).

> +#
> +BR2_CMAKE_VERSION_MIN = 3.1
> +
> +BR2_CMAKE ?= cmake
> +ifeq ($(call suitable-host-package,cmake,\
> +	$(BR2_CMAKE) $(BR2_CMAKE_VERSION_MIN)),)
> +BR2_CMAKE = $(HOST_DIR)/usr/bin/cmake
> +BR2_CMAKE_HOST_DEPENDENCY = host-cmake
> +endif
> diff --git a/support/dependencies/check-host-cmake.sh b/support/dependencies/check-host-cmake.sh
> new file mode 100755
> index 0000000..ec918c7
> --- /dev/null
> +++ b/support/dependencies/check-host-cmake.sh
> @@ -0,0 +1,28 @@
> +#!/bin/sh
> +
> +candidate="${1}"
> +version_min="${2}"
> +
> +major_min="${version_min%.*}"
> +minor_min="${version_min#*.}"
> +
> +cmake=`which ${candidate}`
> +if [ ! -x "${cmake}" ]; then
> +    # echo nothing: no suitable cmake found
> +    exit 1
> +fi
> +
> +version="$(${cmake} --version | head -n1 | cut -d' ' -f3)"
> +major="${version%.*}"
> +minor="${version#*.}"

 I have cmake version 3.6.1 which puts major at 3.6 and minor at 1....

 How about:

version="$(${cmake} --version | head -n1 | cut -d' ' -f3 | cut -d. -f1,2)"

(tested).


 Otherwise, looks good to me.

 Regards,
 Arnout

> +
> +if [ ${major} -gt ${major_min} ]; then
> +    echo "${cmake}"
> +else
> +    if [ ${major} -eq ${major_min} -a ${minor} -ge ${minor_min} ]; then
> +        echo "${cmake}"
> +    else
> +        # echo nothing: no suitable cmake found
> +        exit 1
> +    fi
> +fi
>