Fixes for macOS

[bradhowes]

• Only install dependencies if necessary.
• Continue on when possible to end of script. Fixes Failed to invoke nproc alias #2.
• Used patch from Infinite loop configuring CMake #4 (comment) to fix open_spiel/CMakeLists.txt
• Ignore empty Python_TARGET_VERSION in open_spiel/python/CMakeLists.txt
• Don't use nproc alias -- just use substitution for right command

[lanctot]

Thank you for your contribution! We are still figuring out the tools and workflow for how to properly integrate code from github pull requests. Once we do, we will come back to take a look at merging this pull request. Expect this to be at lease one week, possibly several. Apologies for the delay.

[kevslinger]

+1, works for me on MacOS 10.14.6 (Thanks @bradhowes !! Saved me a lot of frustration).