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

Let's see what the clang bots think of it.

We want to use it.

Makes more sense given what this prints, err, dumps.

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 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.

This only counts recursions right now.  Good start.

Hopefully...
Fixes https://github.com/behdad/harfbuzz/issues/429

Fixes bots.

Link libharfbuzz.so with -lm.  Surprising that we survived without
it so far!

https://github.com/behdad/harfbuzz/pull/555

https://github.com/behdad/harfbuzz/issues/455

Hopefully fixes Travis faliure?
https://github.com/behdad/harfbuzz/pull/413#issuecomment-278481882
https://travis-ci.org/khaledhosny/harfbuzz/jobs/199785332#L1065

API changes:
- If NDEBUG is defined, define HB_NDEBUG
- Disable costlier sanity checks if HB_NDEBUG is defined.

In 1.2.3 introduced some code to disable costly sanity checks if
NDEBUG is defined.  NDEBUG, however, disables all assert()s as
well.  With HB_NDEBUG, one can disable costlier checks but keep
assert()s.

I'll probably add a way to define HB_NDEBUG automatically in
release tarballs.  But for now, production systems that do NOT
define NDEBUG, are encouraged to define HB_NDEBUG for our build.