The... the origin of the problem is that MacOS itself is defining this header...?
/Library/Developer/CommandLineTools/SDKs/MacOSX15.2.sdk/usr/include/sys/cdefs.h:874:9: note: this is the location of the previous definition
#define STDC_WANT_LIB_EXT1 1
... huh. Why is this happening to me. What did I do?!