Linkgraph: Enable MCF custom allocator only for GCC 4.9+

Earlier compilers don't use std::allocator_traits to query allocators.

config.lib

@@ -1413,6 +1413,11 @@ make_compiler_cflags() {
 			flags="$flags -Wno-free-nonheap-object"
 		fi
 
+		if [ $cc_version -ge 49 ]; then
+			# Enable use of C++11 custom allocators
+			CFLAGS="$CFLAGS -DCUSTOM_ALLOCATOR"
+		fi
+
 		if [ "$enable_lto" != "0" ]; then
 			# GCC 4.5 outputs '%{flto}', GCC 4.6 outputs '%{flto*}'
 			has_lto=`$1 -dumpspecs | grep '\%{flto'`