clear out TMP_ cmake variable after FFMPEG use

The CMake TMP_ variable is set and visible in the CMake GUI.  This unsets the variable after use.  In addition, it fixes some display settings for messages and displays the FFMPEG information after it's found.

The build worked just fine for me, however, just one note about this CMake file.  FFMPEG_LIBRARY_DIR does not seem to be set to anything for me.  Everything worked but this appears to potentially be an unnecessary variable or one that is not found through Linux at least.

Notes: Only tested on Linux

Thanks for the new CMake build system.