Merge branch 'ab/auto-detect-zlib-compress2'

The build procedure has been taught to notice older version of zlib
and enable our replacement uncompress2() automatically.

* ab/auto-detect-zlib-compress2:
  compat: auto-detect if zlib has uncompress2()

diff --cc Makefile
Simple merge

diff --cc cache.h
Simple merge

diff --cc config.mak.uname
Simple merge

diff --cc git-compat-util.h
Simple merge