Game runs pretty close to fine, needed to use system libpng and haven't
attempted to use ffmpeg, however.
I'm seeing in the 50fps range in gameplay with no performance tweaks
but having set -mtune=cortex-a72. There's a lot of perf opportunities,
so consistent 60 is certainly within reason without too much pain.
This attempts to organize all needed items and places appropriate defines
in a single location. Redundant comparisons/defines were removed when noticed.
A few caveats, however:
* This may be better targeted towards the 5_1_0 branch instead of master right now.
* Cmake will run a little slower on configuration/generation now. That's due to the sanity checks it runs.
* There are some more checks to be added later, but this should be a solid start.
Thanks to the [libical](https://github.com/libical/libical) project for inspiration.
Colby Klein