Set Zlib as optional requirement for BINSASCII extension

ZLIB_CRC32 support is enabled only if ZLIB_LIBRARIES is set.
Merge branch 'zlib-optional-for-binascii'

* zlib-optional-for-binascii:
  Set Zlib as optional requirement for BINSASCII extension
Add USS_SYSTEM_* options to control which dependent libraries are used.

Decouple extension requirements from system libraries availability. In
other word, it is possible to directly specify the requirement
of each extension without relying on "find_package", "find_library" or
"find_file".

For sake of consistency, move "find_*" system introspection call from
main "CMakeLists.txt" to "ConfigureChecks.cmake"

Fix compilation of pyexpat and _elementtree extension when building
against implementation of expat provided by python.
Merge branch 'tweak-extensions-requirements'

* tweak-extensions-requirements:
  Add USS_SYSTEM_* options to control which dependent libraries are used.
Fix undefined "_environ" symbol link MacOSX error specific to shared build.

This commit fix the MacOSX shared build that was broken as a side effect
of the removal (commit 375f0fca) of the setresgid/setresuid hack.
Merge branch 'fix-macosx-environ-link-error'

* fix-macosx-environ-link-error:
  Fix undefined "_environ" symbol link MacOSX error specific to shared build.
Configure python packageconfig file
Mirror regular build system installing "install-sh" and python static lib
Tweak generated Makefile to match one generated by regular python build

More work is required to ensure it is also properly generated on MacOSX. To
help, a small script named "confall.py" has been created:

   See https://gist.github.com/jcfr/5111758

The config dump generated by confall.py for both an installed tree obtain
using a autoconf system or a CMake system can easily be compared to
further tune the Makefile configuration.

Having the Makefile properly configured seems rather critical to allow
dependent package to be build properly:
  http://bugs.python.org/issue3588
  http://bugs.python.org/issue16848