Was writing source font's offsets. Ouch.

To fix this warning:

cc1: warning: command line option ‘-fno-threadsafe-statics’ is valid for C++/ObjC++ but not for C

Even in libharfbuzz-subset, we still will handle our own threadsafety
for statics.

We use calloc() to allocate simple structs and set them to zero.
For bulk memory, use malloc() that doesn't zero.

Such that hmtx accelerator can be used from libharfbuzz-subset.

sanitizer.sanitize never return nullptr.

We want to use it instead of reinventing hashmap...

Also, don't change linker for ICU since libicu also uses libstdc++.

Also improve wording a bit and fix make check in --with-libstdc++=yes.

Since this is a convenience library, it's not actually linked. Or so I think.
Hoping this might fix this Appveyor failure:

https://ci.appveyor.com/project/harfbuzz/harfbuzz/build/job/9884jyb5vryja9pw

Making all in hb-ucdn
make[4]: Entering directory '/c/projects/harfbuzz/src/hb-ucdn'
  CC       libhb_ucdn_la-ucdn.lo
  GEN      libhb-ucdn.la
/bin/sh: libhb_ucdn_la-ucdn.lo: command not found
make[4]: *** [Makefile:404: libhb-ucdn.la] Error 127
make[4]: Leaving directory '/c/projects/harfbuzz/src/hb-ucdn'
make[3]: *** [Makefile:2548: all-recursive] Error 1
make[3]: Leaving directory '/c/projects/harfbuzz/src'
make[2]: *** [Makefile:1382: all] Error 2
make[2]: Leaving directory '/c/projects/harfbuzz/src'
make[1]: *** [Makefile:486: all-recursive] Error 1
make[1]: Leaving directory '/c/projects/harfbuzz'
make: *** [Makefile:418: all] Error 2

Fixes https://github.com/harfbuzz/harfbuzz/pull/776

This one might do it...

I broke this in c31fcf4c

If we don't have visibility, make NullPool static.

Better fix for https://github.com/harfbuzz/harfbuzz/pull/775

Hopefully it...
fixes https://github.com/harfbuzz/harfbuzz/issues/740