Install Ruby mapscript in sitearchdir instead of archdir.

This change complements #4750. Installing the Ruby mapscript library in /usr/lib/ruby/<version>/<arch> is wrong, while Debian packages installed modules there this is not correct for MapServer upstream.

The installation directory is changed to /usr/local/lib/ruby/site_ruby/<version>/<arch>, quite similar to the Perl mapscript installation directory /usr/local/lib/perl/5.14.2.

The Debian packages modify CMakeLists.txt to use vendorarch instead to install in the /usr prefix instead of /usr/local.