After r255321, clang uses libc++ by default. This leads to a lot of

errors when you enable WITH_GNUCXX to build libstdc++, since it will
include C++ headers from the libc++ installation under ${WORLDTMP}, and
those are not compatible with libstdc++ at all.

To fix this, add -stdlib=libstdc++ to CXXFLAGS when building libstdc++
(and its companion libsupc++) with clang.

Approved by:	re (delphij)

gnu/lib/libstdc++/Makefile

@@ -636,3 +636,7 @@ CLEANFILES+=	${VERSION_MAP}
 
 # Filter out libc++-specific flags, and -std= flags above c++98 or gnu++98.
 CXXFLAGS:=	${CXXFLAGS:N-stdlib=libc++:N-std=c++[01][13x]:N-std=gnu++[01][13x]}
+
+.if ${COMPILER_TYPE} == "clang"
+CXXFLAGS+=	-stdlib=libstdc++
+.endif

gnu/lib/libsupc++/Makefile

@@ -56,3 +56,7 @@ VERSION_MAP=	${.CURDIR}/Version.map
 
 # Filter out libc++-specific flags, and -std= flags above c++98 or gnu++98.
 CXXFLAGS:=	${CXXFLAGS:N-stdlib=libc++:N-std=c++[01][13x]:N-std=gnu++[01][13x]}
+
+.if ${COMPILER_TYPE} == "clang"
+CXXFLAGS+=	-stdlib=libstdc++
+.endif