Allow switching between native (for instance osxappinfo) and generic (freedesktop-based) appinfo implementation at build time

Submitted by Mihai Moldovan

Description

This patchset lets users select the appinfo implementation at build time.

The native OS X (appbundle-based) appinfo implementation is not good enough for all cases. Especially when using GTK+ compiled against X11, having the generic appinfo implementation is more useful.

Also, I have fixed up installing missing headers for the win32appinfo implementation and made that switchable as well - even though the generic appinfo implementation currently won't work on Windows.

The default behavior is retained - by default glib will select the native appinfo implementation as before if the new configure switch is not passed.