whitespaces

Replace PLATFORM with RUBY_PLATFORM and respect other platforms, too. Fixes http://rubyforge.org/tracker/index.php?func=detail&aid=28740&group_id=300&atid=1223

The target objects stored in the accelerator table are currently
private. Therefore FXRbAccelTable::markfunc() doesn't mark them as used.
As a workaround the objects are additionally stored in @targets Hash.

determine which shared library to load.

If RUBY_PLATFORM says we're on Windows, this loader will look under
a version-specific directory to try to load the extension. For other
platforms, it will just load the appropriate shared library from
the current directory.