cmake python config bugs fixed

[ahundt]

• Python and pybullet remain optional with these changes.
• A specific python version can now be selected by setting PYTHON_VERSION_PYBULLET.
• The python library version must now match the interpreter version exactly.
• If all required python dependencies are found, pybullet is now enabled by default.

Changes incorporate the following BSD licensed cmake code:

https://github.com/BVLC/caffe/blob/32bf5c7ad804ad683aa5ea9382209e9284451e5f/CMakeScripts/FindNumPy.cmake
https://github.com/NikolausDemmel/CMake/pull/2/files
https://github.com/Kitware/CMake/blob/86578eccf2e82286248796bad1032cd0e3a5e1e2/Modules/SelectLibraryConfigurations.cmake

This addresses many of the cmake issues blocking #1107

[erwincoumans]

Thanks for the work on this. I will test it on a few platforms before merging.
I think it is better to disable the BUILD_PYBULLET by default, unless the user manually enables it, since I really want Bullet to build out-of-the-box, undisturbed by broken python installations. We can leave it switched on if Python versions match for now. If people report broken builds, we switch it off.

[ahundt]

Okay, I tried to be aware of that concern. It should try and ensure if any problems are detected python automatically switches off, standard warnings are printed, and the cmake configuration + build continues & succeeds without python.

[erwincoumans]

Thanks for the contribution, looks good to me!

[erwincoumans]

People report issues with cmake and python, and the continuous integration also fails.
#1322
We need to have a more reliable way for cmake to locate python