* 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.
