GNUMakefile: Explicitly ask for stdlib=libc++ because clang downgrades the version silently if you use backward compatibility mode with OSX 10.5 and then shrink_to_fit() doesn't work.

Config.macosx: Support for shrink_to_fit() is claimed to be in 10.7 and later, but 10.7 and 10.8 both throw this huge error:
https://paste.factorcode.org/paste?id=3872

Fixes #1624.

GNUmakefile

@@ -12,7 +12,7 @@ ifdef CONFIG
 		-DFACTOR_GIT_LABEL="$(GIT_LABEL)" \
 		$(SITE_CFLAGS)
 
-	CXXFLAGS += -std=c++11
+	CXXFLAGS += -std=c++11 -stdlib=libc++
 
 	ifdef DEBUG
 		CFLAGS += -g -DFACTOR_DEBUG

vm/Config.macosx

@@ -1,6 +1,6 @@
 include vm/Config.unix
 #SITE_CFLAGS +=
-SITE_CFLAGS += -mmacosx-version-min=10.5
+SITE_CFLAGS += -mmacosx-version-min=10.9
 
 PLAF_DLL_OBJS += vm/os-macosx.o vm/mach_signal.o vm/mvm-unix.o
 PLAF_MASTER_HEADERS += vm/os-macosx.hpp vm/mach_signal.hpp