devtools: reset compilation flags for each target

Same idea than overriding PATH and PKG_CONFIG_PATH, it can be quite
useful to override compilation flags like CFLAGS, CPPFLAGS and LDFLAGS
for cross compilation or libraries that won't provide a pkg-config file.

Fixes: 2722367412 ("devtools: load target-specific compilation environment")

Signed-off-by: David Marchand <david.marchand@redhat.com>

devtools/test-meson-builds.sh

@@ -36,12 +36,18 @@ fi
 
 default_path=$PATH
 default_pkgpath=$PKG_CONFIG_PATH
+default_cppflags=$CPPFLAGS
+default_cflags=$CFLAGS
+default_ldflags=$LDFLAGS
 
 load_env () # <target compiler>
 {
 	targetcc=$1
 	export PATH=$default_path
 	export PKG_CONFIG_PATH=$default_pkgpath
+	export CPPFLAGS=$default_cppflags
+	export CFLAGS=$default_cflags
+	export LDFLAGS=$default_ldflags
 	unset DPDK_MESON_OPTIONS
 	command -v $targetcc >/dev/null 2>&1 || return 1
 	DPDK_TARGET=$($targetcc -v 2>&1 | sed -n 's,^Target: ,,p')