Allow building libc++ with any custom -std=xxx flag. Note that using

-std=gnu++98 will give a lot of warnings with clang, so please use
-std=c++0x or higher, or use NO_WERROR.

MFC after:	1 week

lib/libc++/Makefile

@@ -53,7 +53,9 @@ cxxrt_${_S}:
 
 WARNS=		0
 CFLAGS+=	-I${HDRDIR} -I${LIBCXXRTDIR} -nostdlib -DLIBCXXRT
+.if !defined(CXXFLAGS) || ${CXXFLAGS:M-std=*}" == ""
 CXXFLAGS+=	-std=c++0x
+.endif
 
 DPADD=		${LIBCXXRT}
 LDADD=		-lcxxrt