build: fix llvm version detection in freebsd-10

In FreeBSD-10, the banner of clang version is "FreeBSD clang version".
Fix regex to detect it.

PR-URL: #11668
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

configure

@@ -576,11 +576,11 @@ def get_version_helper(cc, regexp):
 
 def get_llvm_version(cc):
   return get_version_helper(
-      cc, r"(^clang version|based on LLVM) ([3-9]\.[0-9]+)")
+    cc, r"(^(?:FreeBSD )?clang version|based on LLVM) ([3-9]\.[0-9]+)")
 
 def get_xcode_version(cc):
   return get_version_helper(
-      cc, r"(^Apple LLVM version) ([5-9]\.[0-9]+)")
+    cc, r"(^Apple LLVM version) ([5-9]\.[0-9]+)")
 
 def get_gas_version(cc):
   try: