pkg.m4 is provided by pkg-config, which is a requirement to build, and this copy is ancient.  Delete it to avoid it being used instead of the host copy.

clang -O3 was completely removing _get_gdef(), causing link
failure when needed from another compilation unit.  Surprisingly,
"extern inline" didn't fix it.

Fixes clang bots as well as fuzzer issue.

There might be a better way to do this, but I couldn't find...

I swear I have no idea why this is needed, but it is...

Used to be ascent+descent.  Whatever. This is more convenient for
upcoming changes.

New API:
+hb_face_collect_unicodes
+hb_face_collect_variation_selectors
+hb_face_collect_variation_unicodes

Sorry for the noise, downstream custom builders.  Please adjust.

New API:
HB_UNICODE_MAX

And use.

To be moved to hb-face.h later.

To be moved to hb-face later.