Change the way C compiler warnings are handled

Change the way C compiler warnings are handled

Before:
 if sufficiently new clang
   add -Weverything and then add a bunch of -Wno-... suppressions

Now:
   if LLPARANOIDP is defined do the -Weverything dance

(Let's see if this passes github's CI, works here on Linux with gcc 11.2.0 and
 clang 13.0.1)