Ruby: install into custom directory

To install Ruby into a custom directory, we use the configure tool with the --prefix flag like this:

  ./configure --prefix=/foo/bar
  make && make install

To install Ruby version 1.9.2 patch level 180 into its own directory:

  ./configure --prefix=/opt/ruby/192p180
  make && make install

To skip the documenation:

  ./configure --prefix=/opt/ruby/192p0 --disable-install-doc
  make && make install

We also want to distinguish the Ruby programs (i.e. commands) that are installed, so we use the --program-suffix flag like this:

  ./configure --prefix=/opt/ruby/192p180 --program-suffix=192p180 --disable-install-doc
  make && make install