remove generated XML mime type

x-sgf.xml represents a *generated* XML mime type and not a proper XML
mime type definition, thus installing it in /usr/share/mime/text will
cause its removal at the next update-mime-database run.

Furthermore, there is no need to provide a mime type definition for
*.sgf files, since shared-mime-info has been providing
application/x-go-sgf for years.
Fix installation on non-Linux and non-Mac Unix OSes

Use the Linux installation paths in any Unix platform different than
Mac OS X (still including Linux), so on those OSes qgo is installed
correctly.

Because of this, shuffle the defines for sounds and translation paths,
so the paths previously used for Linux only are now used for anything
different than Windows and Mac OS X.
Merge pull request #30 from pinotree/mime

remove generated XML mime type
Merge pull request #31 from pinotree/unix-install

Fix installation on non-Linux and non-Mac Unix OSes