Interface for hb_subset, skeleton for the hb-subset cli, and basic testing rigging.

At any position, if state is not zero, mark unsafe-to-break before,
unless we can reason it safe.

At any position, if there's an action entry for end-of-text, mark
unsafe to break.

Also changes buffer diff impl to allow for flag differences as long
as the buffer glyph flags are superset of reference glyph flags.

With this, all MORX tests pass.

tweak test failure output. write to a temp file not stdout. test still fails because expected is not just an identical copy of input

Add basic directory structure for subsetter integration tests. Plus a utility for generating expected output files.

See comment.

With this, MORX-20 passes if I turn --verify off.  Our unsafe-to-break
logic is currently broken in presence of end-of-text actions.  That's,
ugh, extra work to fix.  Let me try...

Fixes https://github.com/harfbuzz/harfbuzz/issues/741

Fixes MORX-21.

Fixes MORX-18, MORX-19, and MORX-22.

Correctly show if building documentation is enabled or not in configure
summary.

Fixes https://github.com/harfbuzz/harfbuzz/issues/741

The atexit() man page indicates that this is expected to behave
in the expected way on unloading of shared libraries.