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
Use AM_MISSING_PROG for ragel and git
Minor
Add Myanmar test case from OpenType Myanmar spec
Remove old cruft
If HAVE_ICU_BUILTIN is defined, use hb-icu Unicode callbacks

Useful for Android / Chrome / etc when ICU is built in the same
library as harfbuzz itself.
[coretext] Add coretext_aat shaper

This is a higher-priority shaper than default shaper ("ot"), but
only picks up fonts that have AAT "morx"/"mort" table.

Note that for this to work the font face's get_table() implementation
should know how to return the full font blob.

Based on patch from Konstantin Ritt.
[coretext] Add hb_coretext_face_create()

Not tested.
wrap definition of free_langs() with HAVE_ATEXIT

...to avoid an unused function warning; see mozilla bug https://bugzilla.mozilla.org/show_bug.cgi?id=984081.
Don't use "register" storage class specifier

Fixes warnings.
https://bugzilla.mozilla.org/show_bug.cgi?id=984081
0.9.27
Merge tag 'tags/0.9.27'

0.9.27

# gpg: error loading `iconv.dll': The specified module could not be found.
#
# gpg: please see http://www.gnupg.org/download/iconv.html for more information
# gpg: keyblock resource `c:/Users/Matt/.gnupg\pubring.gpg': file open error
# gpg: Signature made 03/19/14 04:13:08 using DSA key ID D3531115
# gpg: Can't check signature: public key not found
Updated VS project version strings for recent merge.