cmake: Use GNUInstallDirs for configurable install directories
cmake: Use GNUInstallDirs for configurable install directories

Use CMake GNUInstallDirs module to provide variables for configurable
install directories.  This is necessary to install libqrencode on amd64
systems where libraries belong in /usr/lib64 rather than /usr/lib.
While at it, make all install directories configurable for user's
convenience.

fukuchi/libqrencode,sballe73/libqrencode,fukuchi/libqrencode,sballe73/libqrencode,sballe73/libqrencode,fukuchi/libqrencode,sballe73/libqrencode