GitLab

* Conditionally include Python in the C++ builds.

Added an option `USE_PYTHON` that allows users to decide to link to Python3 or not.

* Also conditionally include this defintion.

* Define `USE_PYTHON` for the Windows builds.

* Remove Python3 reference in CMake test as it's no longer required.

* Accidentally removed the closing > from the decl.

* Also add `USE_PYTHON` when building the separate image library on Windows.

* Also conditionally include this depening on USE_PYTHON.

* Go back to require Python for this example.

* Find Python in the hello world example.

* Add a paragraph documenting the `USE_PYTHON` option.
Co-authored-by: Prabhat Roy <prabhatroy@fb.com>