meson-package: prevent cmake find_package() picking up host libraries

The meson build system uses (1) pkg-config and (2) cmake find_package to locate
dependencies and stops only if a package is found. Cmake uses a toolchain file
that is generated by meson from the existing configuration [1].

The cmake toolchain file misses CMAKE_FIND_ROOT_PATH and CMAKE_FIND_ROOT_PATH_*
settings, and find_package() falls back to the default behaviour and looks for
dependencies on the build machine (the Buildroot HOST_DIR).

Use a feature introduced in meson 0.56.0 [2] to pass the Buildroot toolchain
file to cmake instead of using a meson-generated one.

Fixes the following build failures:
http://autobuild.buildroot.org/results/056/05673ed04c6f044f1b56b9d5342d61653be43a18/
http://autobuild.buildroot.org/results/f0a/f0a9e719114f19dc9d20622ed85dd4f8e968c20f/
http://autobuild.buildroot.org/results/527/527ee199813abbacd61c3fa32b517ea60af60659/

[1[ see mesonbuild/cmake/toolchain.py
[2] see https://mesonbuild.com/CMake-module.html#cross-compilation

Signed-off-by: Andreas Ziegler <br015@umbiko.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 6ab8b1e263)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/misc/cross-compilation.conf.in

@@ -26,6 +26,9 @@ needs_exe_wrapper = true
 sys_root = '@STAGING_DIR@'
 pkg_config_libdir = '@STAGING_DIR@/usr/lib/pkgconfig:@STAGING_DIR@/usr/share/pkgconfig'
 pkg_config_static = '@STATIC@'
+# enable meson build to pass a toolchain file to cmake
+cmake_toolchain_file = '@HOST_DIR@/share/buildroot/toolchainfile.cmake'
+cmake_defaults = false
 
 [host_machine]
 system = 'linux'