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.

Saves some sweet time and binary size!

This adds to the autotools build system so that the (experimental)
DirectWrite support for HarfBuzz is built (and dist'ed).

This moves all the source listings in src/Makefile.am,
src/hb-ucdn/Makefile.am and util/Makefile.am into separate Makefile
snippets, so that they may be shared between different Makefile-based
build systems, such as NMake for Visual Studio.

Currently fails.  Part of
https://bugs.freedesktop.org/show_bug.cgi?id=93275