clang-wrapper: Set $CXX to clang

This is useful for non-Autoconf-based packages, since GNU Make's
default for CXX is "g++".  (The CC default is "cc" so should work fine
with Clang already.)

pkgs/build-support/clang-wrapper/setup-hook.sh

@@ -31,3 +31,6 @@ fi
 if test -n "@coreutils@"; then
     addToSearchPath PATH @coreutils@/bin
 fi
+
+: ${CXX:=clang++}
+export CXX