The only way is to use raw lines for your platform, something like
this for Unix:
OVERRIDES=foo.o
BEGINRAW[Makefile(unix)]
foo.o: foo.c
$(CC) $(CFLAGS) -O3 -c -o $@ $<
ENDRAW[Makefile(unix)]
The reason for this is that as soon as you want to add compiler
specific flags,
Is it possible in the unified build system to apply certain compiler (or
linker) flags only to a specific file or set of files? This could make
some scenarios easier when one is willing to patch the tree (e.g., build
some things with -O3 and others with -O0 -ggdb3).
Given that the unified build
