Apply glib backports

Backporting some commits from glib to xdgmime (see MR 2029: Updating xdgmime).

This patch set include the following glib commits:

• a1bfe899: Don't give up too early when collecting mime types : !10
• 14e46ca2: Don't compile some unused functions in gio/xdgmime/ : !15 (closed)
• e7822bd3: Silence an uninitialize variable warning : !11 (merged)
• e4d18133: xdgmime: Finer handling for cases where mmap() is not available : !13
• b765c795: gio/xdgmime/: LGPLv2+ -> LGPLv2.1+: !12
• 38869ece: xdgmime: Prevent infinite loops from badly-formed MIME registrations: !14 (merged)
• fa6f4553: xdgmime: Add xdg_mime_set_dirs() method to override XDG envvars : !16 (merged)