Matched IlmImf cmake build to the autotools one.
 - Use pkgconfig to find IlmBase
 - Temporary deactivated everything OpenEXR but IlmImf
 - Added OpenEXR.pc
 - Aded 2_0 suffix to IlmImf
 - Always add b44ExpLogTable as a dependency
 - Install missing ImfMisc.h and ImfPartHelper.h
Define include dirs in main CMakeLists.txt