Build: More GNUInstallDirs improvements

Build: More GNUInstallDirs improvements

These improvements enable build systems to use GNUInstallDirs to define
custom directory variables.

- The set_dir() macro was renamed to GNUInstallDirs_set_install_dir(),
  in keeping with the module's established macro naming convention.

- Rather than detecting whether the prefix has changed, the new
  GNUInstallDirs_set_install_dir() macro instead examines whether the
  default for the variable in question has changed.  This allows for
  more flexibility, since build systems may decide to change the
  defaults based on factors other than the prefix.  It also enables the
  macro to work properly outside of the module.

- The module now performs directory variable substitution within the
  body of GNUInstallDirs_get_absolute_install_dir().

- The JAVADIR variable is no longer included in GNUInstallDirs.  That
  directory is not part of the GNU spec, and it turns out that various
  operating systems use different conventions for the location of Java
  classes.  Instead, the variable is now implemented in our build
  system as a demonstration of the aforementioned GNUInstallDirs
  enhancements.