gtk/gtkborderimage.c, gtk/gtklabel.c and gtk/gtkstyleproperty.c call
round() and/or rint(), which was only available in C99 compilers.

This adds the inclusion of the fallback implementation (gtk/fallback-c89.c)
to define these functions if they are not initially made available by the
compiler.

Also remove the rint() implementation in gtk/gtklabel.c as it is now in
the fallback implmentation.