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.