Have the Makefile handle the MozJPEG dependency if it's not on the user's system.

[pope]

This provides a convenience to users who don't have MozJPEG installed to have the Makefile download, configure, and compile the libjpeg.a dependency. As a result, the AppVeyor and Travis-CI scripts are simpler as are the instructions in the README.

The MozJPEG process is broken out into two include files. The first one tries to find MozJPEG on the user's system. If it finds it (or if the user provided the location of MOZJPEG_PREFIX), then the build process will be set up to use that.

If the script wasn't able to find MozJPEG, then the other include file is activated. This include provides all of the rules for downloading, configuring and building MozJPEG in the source.