[subset] Check for the Null table and not nullptr to detect failure to find a table in glyf accelerator.

But keep use of cmake idiomatic way of making inlines hidden

Something weird seems to be going on. Just kill it.

Fixes https://github.com/harfbuzz/harfbuzz/issues/888

They are handled in our common header.

Bad bad bad bad code. Don't do that. If compiler's not happy, understand why.

This reverts commit b9dcbb1f.

This is gross. Should be only done if a specific macro is defined. Not the other way
around. This fails my build as autotools companions where not added anyway.

CMake: hide symbols by default on non-MSVC, define export attribute

This covers all built libraries now and both static and shared builds.