meson.eclass: add _meson_move_flags function

This is used to move flags from CC to CFLAGS, typically during a
multilib build.

eclass/meson.eclass

@@ -198,6 +198,25 @@ meson_use() {
 	usex "$1" "-D${2-$1}=true" "-D${2-$1}=false"
 }
 
+# @FUNCTION: _meson_move_flags
+# @INTERNAL
+# @USAGE: PROG FLAGS
+# @DESCRIPTION:
+# Moves extra arguments from PROG to FLAGS.
+# For example:
+# CC="gcc -m32" -> CC="gcc" CFLAGS="-m32"
+_meson_move_flags() {
+	local prog=${1}
+	local flags=${2}
+	local x=( ${!prog} )
+	if [[ -n ${x[0]} ]]; then
+		export ${prog}=${x[0]}
+	fi
+	if [[ -n ${x[1]} ]]; then
+		export ${flags}="${x[@]:1}${!flags:+ }${!flags}"
+	fi
+}
+
 # @FUNCTION: meson_src_configure
 # @DESCRIPTION:
 # This is the meson_src_configure function.
@@ -214,6 +233,18 @@ meson_src_configure() {
 		--wrap-mode nodownload
 		)
 
+	# Prevent multilib flags from leaking across ABIs
+	local -x BUILD_CFLAGS=${BUILD_CFLAGS}
+	local -x BUILD_CXXFLAGS=${BUILD_CXXFLAGS}
+
+	# Move multilib flags from CC to CFLAGS
+	local -x CC=$(tc-getCC) CFLAGS=${CFLAGS}
+	_meson_move_flags CC CFLAGS
+
+	# Move multilib flags from CXX to CXXFLAGS
+	local -x CXX=$(tc-getCXX) CXXFLAGS=${CXXFLAGS}
+	_meson_move_flags CXX CXXFLAGS
+
 	if tc-is-cross-compiler; then
 		_meson_create_cross_file || die "unable to write meson cross file"
 		mesonargs+=( --cross-file "${T}/meson.${CHOST}" )