Update zlib from version 1.2.7 to 1.2.8 on vendor branch
Merge branch 'vendor/ZLIB'
zlib: Upgrade from version 1.2.7 to 1.2.8

There were four releases since the last update.  These bullets were
selectively removed from the Changelog:

Changes in 1.2.7.1 (24 Mar 2013)
- Replace use of unsafe string functions with snprintf if available
- Fix gzgetc undefine when Z_PREFIX set [Turk]
- Fix bug in 'F' mode for gzopen()
- Add inflateGetDictionary() function
- Correct comment in deflate.h
- Delete "--version" file if created by "ar --version" [Richard G.]
- Remove unused variable in infback9.c
- Fix argument checks in gzlog_compress() and gzlog_write()
- Clean up the usage of z_const and respect const usage within zlib
- Avoid shift equal to bits in type (caused endless loop)
- Fix unintialized value bug in gzputc() introduced by const patches
- Fix memory allocation error in examples/zran.c [Nor]
- Fix bug where gzopen(), gzclose() would write an empty file
- Fix bug in gzclose() when gzwrite() runs out of memory
- Add note to contrib/blast to use binary mode in stdio
- Fix comparisons of differently signed integers in contrib/blast
- Check for invalid code length codes in contrib/puff
- Fix serious but very rare decompression bug in inftrees.c
- Update inflateBack() comments, since inflate() can be faster
- Use underscored I/O function names for WINAPI_FAMILY
- Add _tr_flush_bits to the external symbols prefixed by --zprefix
- Fix casting error in contrib/testzlib/testzlib.c
- Fix types in contrib/minizip to match result of get_crc_table()
- Simplify contrib/vstudio/vc10 with 'd' suffix
- Fix typos in the use of _LARGEFILE64_SOURCE in zconf.h
- Add gzvprintf() as an undocumented function in zlib
- Remove runtime check in configure for four-byte integer type
- Add casts and consts to ease user conversion to C++
- Do not return Z_BUF_ERROR if deflateParam() has nothing to write

Changes in 1.2.7.2 (13 Apr 2013)
- Change check for a four-byte type back to hexadecimal
- Fix typo in win32/Makefile.msc
- Add casts in gzwrite.c for pointer differences

Changes in 1.2.7.3 (13 Apr 2013)
[Not applicable]

Changes in 1.2.8 (28 Apr 2013)
- Do not force Z_CONST for C++
- Correct spelling error in zlib.h
Merge branch 'master' into gsocdrm_github