Installation place of cmake and pkgconfig files

[papadop]

The base directory for cmake and pkgconfig files is CMAKE_INSTALL_DATAROOTDIR which is set by default to /usr/share on linux. The documentation says that it is better to use GNUInstallDirs instead. Indeed, with the current cmake, cmake and pkgconfig files end up in /usr/share/{cmake,pkconfig} whereas they should end up in /usr/lib64/{cmake,pkconfig} instead (lib64 or lib may depend on the linux distribution).