illumos gate

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.