LINK should not need CPPFLAGS
Got warning with clang:
clang-13: warning: argument unused during compilation: '-nostdinc' [-Wunused-command-line-argument]
Investigating the issue, I found this error is actually from rule to link:
$(PROG): $(OBJS) $(LINK.c) $(OBJS) -o $@ $(LDLIBS) $(POST_PROCESS)
And LINK.c is defined to include $(CPPFLAGS), the gcc does not mind, but clang does (and rightfully so, I think). Move CPPFLAGS out from LINK.* macros.
Testing done: full nightly run.