Allow hb_atexit redefinition

[jesusdesantos]

Our library (using harfbuzz internally) exposes memory callbacks to our clients to implement custom allocators and leak detectors.

We also expose a Finalize point in the library where all allocations not released are counted as leaks.

Harfbuzz is doing a few allocations that are expected to be freed at atexit time (or never, depending on the implementation). These allocations count as leaks in our library. To solve this, we modified Harfbuzz to allow pre-defining hb_atexit (before hb.hh defines it).

We redefine hb_atexit to push each function into an internal list. These functions are manually invoked at Finalize time.

The changes to the code are very simple, not sure if aligned with the design of the library but I would love to have this upstream.

+#ifndef hb_atexit
 #  if !HB_USE_ATEXIT
 #    define hb_atexit(_) HB_STMT_START { if (0) (_) (); } HB_STMT_END
 #  else /* HB_USE_ATEXIT */
 #    ifdef HAVE_ATEXIT
 #      define hb_atexit atexit
 #    else
        template <void (*function) (void)> struct hb_atexit_t { ~hb_atexit_t () { function (); } };
 #      define hb_atexit(f) static hb_atexit_t<f> _hb_atexit_##__LINE__;
+#    endif