bug#45771: 28.0.50; GMP not showing in system-configuration-feature

[Eli Zaretskii <eliz@gnu.org>]

> From: Phillip Lord <phillip.lord@russet.org.uk>
> Date: Sun, 10 Jan 2021 18:08:43 +0000
> 
> The string "GMP" is not showing in system-configuration-feature on Emacs
> master branch, even when libgmp has been configured and is reported to
> be in use by configure. Confirmed on builds on both Ubuntu and
> Windows.

I think this is because:

  for opt in XAW3D XPM JPEG TIFF GIF PNG RSVG CAIRO IMAGEMAGICK SOUND GPM DBUS \
    GCONF GSETTINGS GLIB NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT \
    LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS X_TOOLKIT OLDXMENU X11 XDBE XIM \
    NS MODULES THREADS XWIDGETS LIBSYSTEMD JSON PDUMPER UNEXEC LCMS2 GMP; do

      case $opt in
	PDUMPER) val=${with_pdumper} ;;
	UNEXEC) val=${with_unexec} ;;
	GLIB) val=${emacs_cv_links_glib} ;;
	NOTIFY|ACL) eval val=\${${opt}_SUMMARY} ;;
	TOOLKIT_SCROLL_BARS|X_TOOLKIT) eval val=\${USE_$opt} ;;
	THREADS) val=${threads_enabled} ;;
	*) eval val=\${HAVE_$opt} ;;
      esac

but we set HAVE_GMP only afterwards:

  if test -z "$GMP_H"; then
    HAVE_GMP=yes
  else
    HAVE_GMP=no
  fi

We need to move the latter before the former.

> As a slightly separate issue, this appears to be the only way of showing
> that libgmp is available for use.

Yes, because if GMP is not available, we have a replacement.