build: Fix option to explicitly disable memory debugging

build: Fix option to explicitly disable memory debugging

The memory management system supports a debug mode that can be enabled
at build time by passing "--enable-mm-debug" to the configure script.
Passing the option will cause us define MM_DEBUG as expected, but in
fact the reverse option "--disable-mm-debug" will do the exact same
thing and also set up the define. This currently causes the build of
"lib/gnulib/base64.c" to fail as it tries to use `grub_debug_malloc()`
and `grub_debug_free()` even though both symbols aren't defined.

Seemingly, `AC_ARG_ENABLE()` will always execute the third argument if
either the positive or negative option was passed. Let's thus fix the
issue by moving the call to`AC_DEFINE()` into an explicit `if test
$xenable_mm_debug` block, similar to how other defines work.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>