Re: pruning stale C language variants [done]

Trip report from the change:

  grepping pkgsrc with wip present, Makefile* for USE_CC_FEATURES,
  discarding comments and confusion, the census is
    122 c99
     48 c11
     17 c17
      2 gnu11
      2 c99 # and alignas/alignof from C11
      1 gnu99      # intptr_t, strdup
  Looking at FORCE_C_STD
     53 gnu99
     36 c99
     24 gnu89
     15 c89
      2 gnu11
      1 gnu99 # src/cli/opt.c uses alloca(3)
      1 gnu99 # alloca
      1 gnu99      # fails with gnu23
      1 gnu99      
      1 gnu90
      1 gnu89 # Uses "old-style-definitions"
      1 gnu89 # Uses "old-style function definitions"
      1 c90
      1 c17
      1 c11 # stdatomic.h
      1 c11


I fixed the one use of c90 to c89, so now c90, c9x, c1x, and c2x are not
used in any Makefile.  Other then FORCE_C_STD, they are also not
implemented in USE_CC_FEATURES.

I did the same thing to gnu9x etc.

I fixed a doc/comment that talked about c90/gnu90 to say 89.