makepkg: unset variables as per !{make,build}flags

Don't just set the flag variables to zero length strings, actually unset
them from the environment. This fixes issues with broken gnu Makefies
that use ?= for assigment, where the presence of a var is enough to make
this condition avoid assignment.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Dan McGee <dan@archlinux.org>

scripts/makepkg.sh.in

@@ -864,14 +864,12 @@ run_function() {
 
 	# clear user-specified buildflags if requested
 	if [[ $(check_option buildflags) = "n" ]]; then
-		CFLAGS=""
-		CXXFLAGS=""
-		LDFLAGS=""
+		unset CFLAGS CXXFLAGS LDFLAGS
 	fi
 
 	# clear user-specified makeflags if requested
 	if [[ $(check_option makeflags) = "n" ]]; then
-		MAKEFLAGS=""
+		unset MAKEFLAGS
 	fi
 
 	msg "$(gettext "Starting %s()...")" "$pkgfunc"