Update for AC_PROG_STDC_CC fix

[law@redhat.com]

  > Isn't something needed for HP/UX to get the compiler into the right 
  > mode?  But yes I agree, even after renaming the macro, there is a 
  > problem with prog_cc_stdc clashes.
True, that would be a problem.  I wouldn't have mentioned the idea of
zapping AM_PROG_CC_STDC except for the fact that I couldn't figure out
if/how it would ever be used.

It seems to me like the various Makefiles associated with gdb use CC,
but I couldn't find the mechanism by which CC was assigned the result
of AM_PROG_CC_STDC.  But apparently by some magic it does happen.

This in turn begs the question -- if we rename AM_PROG_CC_STDC to
GDB_PROG_CC_STDC will the value computed by GDB_PROG_CC_STDC still
be used to initialize CC in the generated Makefile?

I can try some experiments, but it would really be better if someone
who understands autoconf/automake better could give me some insight
into what unintended consequences we might have due to the name change.

jeff