allowing fallback to clang on freebsd when needed

configure

@@ -861,6 +861,16 @@ then
     CFG_DISABLE_JEMALLOC=1
 fi
 
+if [ $CFG_OSTYPE = unknown-freebsd ]
+then
+  CFG_FREEBSD_GCC_VERSION=$("CFG_GCC" --version 2>&1)
+  if [ $? -ne 0 ]
+  then
+    step_msg "GCC not installed on FreeBSD, forcing clang"
+    CFG_ENABLE_CLANG=1
+  fi
+fi
+
 # OS X 10.9, gcc is actually clang. This can cause some confusion in the build
 # system, so if we find that gcc is clang, we should just use clang directly.
 if [ $CFG_OSTYPE = apple-darwin -a -z "$CFG_ENABLE_CLANG" ]