require g2c during build, remove duplicate C code for compression from g2, use g2c directly

[edwardhartnett]

Currently we have some C code copied from g2c in the g2 project: g2cjpeg2000.c and g2cpng.c.

These are both in the g2c library. We need to make the g2c library mandatory in the build, and use these functions from g2c, instead of having duplicate copies of the code.

We then need to add AEC to the g2 library, using it's implementation in the g2c library.

Also, whenever we call C, we need to make sure we are using the ISO standard C/Fortran API, not relying on the underscore trick, which is not standard.

[edwardhartnett]

Does anyone have any code to check in for this?

I would like to reassign this to @AlysonStahl-NOAA who is now working on g2/g2c...