build: Build backend libraries before the backend modules themselves

https://bugzilla.gnome.org/show_bug.cgi?id=711544
build: Use AS_HELP_STRING instead of the deprecated AC_HELP_STRING

https://bugzilla.gnome.org/show_bug.cgi?id=711544
build: Update configure.ac as suggested by autoupdate

Thanks, autoupdate.

https://bugzilla.gnome.org/show_bug.cgi?id=711544
build: Remove unused variables from configure.ac

https://bugzilla.gnome.org/show_bug.cgi?id=711544