core: don't build host-cmake if it is available on the build host

core: don't build host-cmake if it is available on the build host

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>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>