Add support for pkg_config

Add support for pkg_config to be consistent with ilmbase and openexr. I'm open to changes here since there are multiple options :
- move installation to CMakeLists.txt in root folder (aside CPack). Caveat is libraries are not accessible here so I would need to export them.
- use a .in file and a configure_file command to mimic OpenEXR install, I went to inline config as you can see.

Also I'm adding all the libraries without paying attention to USE_xxx, this can be improved.