fix: fold [build].defines into cflags/cxxflags before P1689 scan #582

Job	Run time
toolchain: musl + llvm	6m 34s
integration: mcpp builds & runs xlings	6m 27s
build + unit tests (linux x86_64, self-host)	7m 52s
toolchain: gcc (cold self-host)	5m 23s
	26m 16s

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