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>