Fix GNUstep build warnings

Fix GNUstep build warnings

* src/nsterm.h ([EmacsWindow orderedIndex]):
* src/nsterm.m ([EmacsWindow orderedIndex]): Implement orderedIndex
for use under GNUstep.
* src/nsmenu.m (free_frame_menubar):
(ns_update_menubar):
([EmacsMenu addSubmenuWithTitle:]):
([EmacsMenu addItemWithWidgetValue:attributes:]): Cast return values
to correct types.
([EmacsMenu fillWithWidgetValue:]): Move variable definition inside
relevant #ifdef block.
([EmacsMenu menuWillOpen:]):
([EmacsMenu menuDidClose:]):
([EmacsMenu confinementRectForMenu:onScreen:]):
([EmacsMenu menu:willHighlightItem:]): New functions to silence build
warnings.
* src/nsfont.m (nsfont_open): Remove pointless fabs call.