Mixing CC and a different CLANG seems like a bad idea

I noticed that, a week after Michael pushed 9ff47ea41 to silence
-Wcompound-token-split-by-macro warnings, buildfarm member sidewinder
is still spewing them.  Investigation shows that it's building with

configure: using compiler=cc (nb4 20200810) 7.5.0
configure: using CLANG=ccache clang

and the system cc doesn't know -Wcompound-token-split-by-macro,
so we don't use it, but the modules that are built into bytecode
still produce the warnings because they're built with clang.

I think this idea of using clang with switches selected for some other
compiler is completely horrid, and needs to be nuked from orbit before
it causes problems worse than mere warnings.  Why did we not simply
insist that if you want to use --with-llvm, the selected compiler must
be clang?  I cannot see any benefit of mix-and-match here.

            regards, tom lane