Give a better error if malloc.c is used without MYMALLOC

The build will die already on errors because of the missing ASSERT
macros (plus get warnings on the implicit declarations, i.e. missing
prototypes), but better to give a clear error.

FWIW, looking at hints/*, only HP-UX of the even remotely
common/current systems seems to use Perl's malloc anymore,
and then only if perlio is NOT being used.

(Other platforms that seemingly use Perl's malloc include ancient
SysVs like SCO 2.3.4, NCR Tower, Tektronix' UTek V, then Unicos and
Unicos/mk of Cray, and NEC's SUPER-UX.)  (OS/2 still probably does
use it, given all the work ilyaz used to pour on it.)

malloc.c

@@ -253,6 +253,10 @@
 #     define PERL_MAYBE_ALIVE	1
 #endif
 
+#ifndef MYMALLOC
+#  error "MYMALLOC is not defined"
+#endif
+
 #ifndef MUTEX_LOCK
 #  define MUTEX_LOCK(l)
 #endif