This expands on #777 to work with jconfig.h properly.
To facilitate these operations, a new macro was added for doing compile tests.
Note that this commit does NOT bring in the other files associated with this version of libjpeg.
At some point, it would be desirable to automatically get the files as part of the cmake configuration step, similar to ffmpeg.
This removes the need for an externally built library, thus officially removing the deprecated Xbox projects.
Other minimal changes were made to ensure consistency. Comments will be made in the PR.
Thanks to @Prcuvu for the initial push.
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.
Jason Felds