Change installed .cmake and .pc files to contain realtive paths

The LIB_DESTINATION and INCLUDE_DESTIONATION variables do hold little value, since one can just override the normal CMAKE variables as needed. Furthermore using CMAKE_INSTALL_FULL_LIBDIR is harmful, since this prevents CMake to use it's logic to install relocatable .cmake files. Also libdir and includedir in pkgconfig files need to be prefixed by either ${exec_prefix} or ${prefix} for the same relocation reasons.