Improve MemoryBarrier() implementation

See thread "[HarfBuzz] compilation error of 0.9.26 with MinGW"
started by Werner.
typo in ucdn's Makefile.am
check hb_set_is_emtpy in test-set.c
Allow bootstrapping without gtk-doc

Seems like configure.ac is already able to handle it.
Only do fallback Hebrew composition if no GPOS 'mark' available

Apparently some modern fonts have proper GPOS mark positioning
tables, but undesirable precomposed forms!  See thread
"Hebrew composition to presentation forms" and:

  http://tex.stackexchange.com/questions/156775/having-trouble-with-vowel-positioning-in-ezra-sil-xelatex

Test case: U+fb1d,05d9,05b4
Merge branch 'master' of git://github.com/behdad/harfbuzz