only probe for python if PYBULLET explicitly specified

[jl]

On Ubuntu 16.04, despite using -DBUILD_PYBULLET=OFF, the build fails with the following error:

CMake Error at build3/cmake/FindPythonLibs.cmake:118 (message):
  Could not find required file 'FindLibPython.py'
Call Stack (most recent call first):
  CMakeLists.txt:317 (FIND_PACKAGE)

If we look at the relevant code in CMakeLists.txt:

FIND_PACKAGE(PythonLibs ${PYTHON_VERSION_STRING} EXACT)

which FATAL_ERRORs out when it encounters this logic in FindPythonLibs.cmake

if(NOT _FIND_LIB_PYTHON_PY)
    message(FATAL_ERROR "Could not find required file 'FindLibPython.py'")
endif()

This pull request changes the build behavior so that the environment is not probed at all for any Python libraries unless BUILD_PYBULLET is explicitly set to ON. The previous behavior was to always probe the environment, then set the default value of BUILD_PYBULLET based on whether the probe succeeds.

With this change, developers not interested in PYBULLET can isolate themselves from problems related to Python build going forward. The downside is folks that want PYBULLET and previously relied on the automagic probing behavior will now need to explicitly turn it ON.

Previous work: #1340

[erwincoumans]

Thank you very much, indeed we shouldn't fail if people don't request Python/pybullet!