Add fallback in case of xdg-utils is not installed

Some distributions may not install xdg-utils by default,
so inform the user in case of xdg-utils is not installed
when the user tries to use any of the xdg-utils commands.
I forget how C works after using Python...
Merge pull request #193 from m45t3r/master

Add fallback in case of xdg-utils is not installed