* configure.in: add AC_HEADER_STDC check.

    * configure.in: add AC_HEADER_STDC check.

        * mmalloc.h: check if STDC_HEADERS instead of __STDC__.

        * mmprivate.h: check if STDC_HEADERS instead of __STDC__.

This change is necessary to build under AIX 3.2.5 w/ cc after Stan's
Oct 24 change.  I'm not convinced that changing mmalloc.h in this way
is such a good thing--if a directory that doesn't use autoconf (or one
that DOES use autoconf but doesn't put all the defines on the command
line), and it includes mmalloc.h, the right thing won't happen.

I believe gdb is the only directory which uses mmalloc, though, so
it should be OK.