Look what an old #macOS SDK in the “Command Line Tools" tree causes on Apple Silicon:
$ clang -o ghba geh.c
In file included from geh.c:14:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/stdlib.h:62:
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/sys/cdefs.h:784:2: error: Unsupported architecture
784 | <…