update to configure bug in Bison 1.31

[Tony Leneis]

        I should have proof read my e-mail and also tested it...  The
function should have been named rpl_malloc(), and it has to exactly
match the prototype for malloc() (since the define in confdefs.h
is apparently done before stdlib.h is included.)

        Perhaps the simplest solution is to just add "#undef malloc"
to the beginning of the mmap() test program.  That works fine on
Tru64 Unix, though I suppose it could potentially be a problem for
other systems.

        Again, the problem was that when the test for a working malloc()
fails, configure adds a line to confdefs.h that redefines malloc to be
rpl_malloc.  Subsequently, any remaining test programs that use malloc()
and include confdefs.h fail.  In bison's case, the mmap() test always
indicates that mmap() doesn't exist or doesn't work, because the linker
can't find a symbol for rpl_malloc.

-Tony