build-style/meson: define PKG_CONFIG for host usage.

PKG_CONFIG environment variable should point to the pkg-config executable
to be used to fetch dependencies that use 'native: true' (indicate to build
against host libraries not cross-compiled ones)

To meet those requirements we set an absolute path to the host
pkg-config since the relative path to pkg-config is taken by the
wrapper.

This currently requires a patch that is a milestone of meson-0.48 that
was generated by the issue:

https://github.com/mesonbuild/meson/issues/1736

common/build-style/meson.sh

@@ -65,6 +65,12 @@ EOF
 		# are set to (C|CXX)FLAGS_host
 		export CFLAGS=${CFLAGS_host} CXXFLAGS=${CXXFLAGS_host}
 
+		# Meson tries to use our wrapped cross-only pkg-config to find
+		# libraries even when 'native: true' (build against the host platform)
+		# is set, so set the PKG_CONFIG variable to tell Meson which pkg-config
+		# it should use when searching for stuff in the build machine
+		export PKG_CONFIG="/usr/bin/pkg-config"
+
 		unset _MESON_CPU_FAMILY _MESON_TARGET_CPU _MESON_TARGET_ENDIAN
 	fi