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.
Add a RunCMake.GNUInstallDirs test with cases covering various install
prefixes. Hard-code the architecture information. Tolerate all
platform-specific LIBDIR values.
Currently the root prefix is not handled well, but verify the current
behavior in the test anyway. This can be addressed with a future
change.
Inspired-by: Alex Turbov <i.zaufi@gmail.com>