build: fix configure script to work with Apple Clang 11

PR-URL: #28071
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann (רפאל פלחי) <refack@gmail.com>

configure.py

@@ -709,7 +709,7 @@ def get_llvm_version(cc):
 
 def get_xcode_version(cc):
   return get_version_helper(
-    cc, r"(^Apple LLVM version) ([0-9]+\.[0-9]+)")
+    cc, r"(^Apple (?:clang|LLVM) version) ([0-9]+\.[0-9]+)")
 
 def get_gas_version(cc):
   try: