libsdl: Fix ${B} != ${S} issues

Fix out of tree builds by remvoing cwd assumption.

(From OE-Core rev: 4a805f74b8a2ac5db27ed8f3c3232d2391a18842)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-graphics/libsdl/libsdl_1.2.15.bb

@@ -57,7 +57,7 @@ do_configure_prepend() {
         # Remove old libtool macros.
         MACROS="libtool.m4 lt~obsolete.m4 ltoptions.m4 ltsugar.m4 ltversion.m4"
         for i in ${MACROS}; do
-               rm -f acinclude/$i
+               rm -f ${S}/acinclude/$i
         done
         export SYSROOT=$PKG_CONFIG_SYSROOT_DIR
 }