build: catch clang that identifies itself as "$VENDOR clang"

The case statement in detect-compiler notices 'clang', 'FreeBSD
clang' and 'Apple clang', but there are other platforms that follow
the '$VENDOR clang' pattern (e.g. Debian).

Generalize the pattern to catch them.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

detect-compiler

@@ -38,13 +38,10 @@ case "$(get_family)" in
 gcc)
 	print_flags gcc
 	;;
-clang)
+clang | *" clang")
 	print_flags clang
 	;;
-"FreeBSD clang")
-	print_flags clang
-	;;
-"Apple LLVM"|"Apple clang")
+"Apple LLVM")
 	print_flags clang
 	;;
 *)