configure: preserve cross-prefix for strip on darwin

Originally committed as revision 23657 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2164,7 +2164,7 @@ case $target_os in
     darwin)
         enable malloc_aligned
         SHFLAGS='-dynamiclib -Wl,-single_module -Wl,-install_name,$(SHLIBDIR)/$(SLIBNAME),-current_version,$(LIBVERSION),-compatibility_version,$(LIBMAJOR) -Wl,-read_only_relocs,suppress'
-        strip="strip -x"
+        strip="${strip} -x"
         FFLDFLAGS="-Wl,-dynamic,-search_paths_first"
         SLIBSUF=".dylib"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME).$(LIBVERSION)$(SLIBSUF)'