GNUInstallDirs: Add special cases for certain prefixes

Teach the module to handle SYSCONFDIR and LOCALSTATEDIR properly if
CMAKE_INSTALL_PREFIX is set to `/` or `/usr` -- i.e. as expected by GNU
Coding Standard (i.e. set SYSCONFDIR to `/etc` and `LOCALSTATEDIR` to
`/var`).  Also if CMAKE_INSTALL_PREFIX is set to /opt/pkg, `SYSCONFDIR`
must be set to `/etc/opt/pkg` and `LOCALSTATEDIR` to `/var/opt/pkg`
according to FHS.