If we have a CLANG compiler, use it in the absense of any preferences.

This only affects development environments on FreeBSD, but hopefully
exposing the Varnish source code to yet another compiler will help
expose bugs.

autogen.des

@@ -5,6 +5,12 @@
 
 set -ex
 
+# Prefer CLANG if we have it, and have not given preferences
+if [ -f /usr/bin/clang -a "x${CC}" = "x" ] ; then
+	CC=clang
+	export CC
+fi
+
 . ./autogen.sh
 
 # autoconf prior to 2.62 has issues with zsh 4.2 and newer