Technically, static const needs an out-of-class definition.  Eg:

  CXXLD    libharfbuzz-subset.la
Undefined symbols for architecture x86_64:
  "OT::FeatureVariationRecord::min_size", referenced from:
      bool OT::GSUBGPOS::subset<OT::PosLookup>(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
      bool OT::GSUBGPOS::subset<OT::SubstLookup>(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
  "OT::Record<OT::LangSys>::min_size", referenced from:
      OT::Script::subset(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
  "OT::IntType<unsigned short, 2u>::min_size", referenced from:
      OT::Script::subset(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
      OT::RecordListOf<OT::Feature>::subset(hb_subset_context_t*) const  in libharfbuzz_subset_la-hb-subset.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status
make[4]: *** [libharfbuzz-subset.la] Error 1
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
Exited with code 2

They are still in the drop list.

Should remove the vector version at some point...

Why does subset plan not have a hb_set_t of glyphs?

Ouch.

Remove Lookup::subset().

Add subset() call for GSUBGPOS struct and its dependencies.
Not hooked up anywhere.

We're going to (finally) use virtual methods for hb_serialize_context_t
customization, so don't need to carry a Serializer template variable
around...  Simplifies code.

We might reintroduce it later, but for now remove, as it was unused.
Some things that should have been in this object (drop_hints, etc)
are already in hb_subset_input_t.  So, for now, keep everything there.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1145

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...