[IMP-dev] IMP CMake builds now default to Python3

[Ben Webb <ben@salilab.org>]

Note that in IMP's develop branch (as of 9f63080a6, committed earlier 
today) CMake will now default to using Python 3 if available:
https://github.com/salilab/imp/commit/9f63080a6

This new logic should make building IMP on systems with multiple Pythons 
(e.g. Macs with Homebrew) much simpler, as CMake should handle this 
without needing to manually set PYTHON_INCLUDE_PATH or PYTHON_LIBRARY 
directly.
If you still want to build IMP with Python 2 on a system that has Python 
3, pass -DUSE_PYTHON2=on to your cmake invocation. (But note that Python 
2 is - very slowly - being phased out so switching to 3 might make 
sense. For example Fedora 30 no longer has Python 2 scikit-learn or 
biopython packages.)
	Ben
--
ben@salilab.org                      https://salilab.org/~ben/
"It is a capital mistake to theorize before one has data."
	- Sir Arthur Conan Doyle