Port to platforms with gtk3 but not webkitgtk3

Port to platforms with gtk3 but not webkitgtk3

I ran into this problem on my Fedora 23 installation;
Emacs configured but did not build when --with-xwidgets was specified.
* configure.ac (HAVE_WEBKIT, HAVE_GIR): Omit unnecessary initializations.
(DOES_XWIDGETS_USE_GIR): New var.
If --with-xwidgets is specified, report an error if not
doable, to be consistent with the other --with options.
Require webkitgtk3 to use Xwidgets, as the Xwidgets code does
not work at all without webkitgtk3.  Simplify use of
EMACS_CHECK_MODULES.  Output message about gobject
introspection only if xwidgets are used.
* etc/NEWS: Users need webkitgtk3, not merely webkit.
* src/xwidget.c (syms_of_xwidget): Don’t worry about HAVE_WEBKIT_OSR,
since this file is no longer compiled if webkitgtk3 is not available.