build-support/meson: explicitly use ambiant CMake during cross compilation

Thanks to Qyriad, it was found out that Nixpkgs was unable to cross
compile when CMake was needed at cross compilation time with Meson
because Meson refuse to use the ambiant CMake.

A simple fix is to always provide the ambiant CMake and trust the
packager to pass the right CMake, otherwise the rest of the build would
probably fail in mysterious ways.

An example of package that required this fix is the Lix, a Nix
implementation, that uses `toml11` discovered via CMake during the Meson
configure phase.

Co-authored-by: Qyriad <qyriad@qyriad.me>
Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

pkgs/build-support/lib/meson.nix

@@ -23,6 +23,10 @@ let
     [binaries]
     llvm-config = 'llvm-config-native'
     rust = ['rustc', '--target', '${stdenv.targetPlatform.rust.rustcTargetSpec}']
+    # Meson refuses to consider any CMake binary during cross compilation if it's
+    # not explicitly specified here, in the cross file.
+    # https://github.com/mesonbuild/meson/blob/0ed78cf6fa6d87c0738f67ae43525e661b50a8a2/mesonbuild/cmake/executor.py#L72
+    cmake = 'cmake'
   '';
 
   crossFlags = optionals (stdenv.hostPlatform != stdenv.buildPlatform) [ "--cross-file=${crossFile}" ];