Clang 9 no longer pretends to be GCC
https://reviews.llvm.org/rL356931
https://reviews.llvm.org/rL366091

--- configure.orig	2019-09-08 15:43:06 UTC
+++ configure
@@ -6464,8 +6464,8 @@ if test "$GCC" = yes && test "$ICC" = no ; then
 	""|2.*|3.*) ;;
 	4.*) gcc4=yes ;;
     esac
-    case "$CC" in
-	*clang) gcc4=yes ;;
+    case `$CC --version 2>/dev/null` in
+	*clang*) gcc4=yes ;;
     esac
 fi
  if test $gcc4 = yes; then