[Bug-XBoard] configure doesn't do the right thing after no malloc.h detected on MacOSX

[Dima Pasechnik]

There is no malloc.h header on MacOSX 10.6,
which is of course not a problem, but engineoutput.c still includes malloc.h
and does not compile because of this.
Presenting it with a dummy (empty, say) malloc.h) allows
the program to compile, build, and work just fine, so this is just a question of
making configure do the right thing in this case.

Just in case,
Dmitrii, a user of xboard for 15 years