* include dir: would only try ${PC_ITPP_INCLUDEDIR}/include:
* added ${PC_ITPP_INCLUDEDIR} as HINT
* dirs: differentiate between HINTS and PATHS
* ITPP_LIBRARY: Now works with CMAKE_BUILD_TYPE=RelWithDebInfo and
debug version of libitpp
* if build type is RelWithDebInfo and both libraries (libitpp and
libitpp-debug) are present, use debug variant.