build: update detect-compiler for newer Xcode version

1da1580e4c (Makefile: detect compiler and enable more warnings in
DEVELOPER=1, 2018-04-14) uses the output of the compiler banner to
detect the compiler family.

Apple had since changed the wording used to refer to its compiler
as clang instead of LLVM as shown by:

  $ cc --version
  Apple clang version 12.0.5 (clang-1205.0.22.9)
  Target: x86_64-apple-darwin20.6.0
  Thread model: posix
  InstalledDir: /Library/Developer/CommandLineTools/usr/bin

so update the script to match, and allow DEVELOPER=1 to work as
expected again in macOS.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

detect-compiler

@@ -44,7 +44,7 @@ clang)
 "FreeBSD clang")
 	print_flags clang
 	;;
-"Apple LLVM")
+"Apple LLVM"|"Apple clang")
 	print_flags clang
 	;;
 *)