configure: Use X11 detection macro from common

https://bugzilla.gnome.org/show_bug.cgi?id=729621

configure.ac

@@ -542,32 +542,7 @@ dnl *** X11 ***
 translit(dnm, m, l) AM_CONDITIONAL(USE_X, true)
 AG_GST_CHECK_FEATURE(X, [X libraries and plugins],
                   [ximagesink], [
-  AC_PATH_XTRA
-  ac_cflags_save="$CFLAGS"
-  ac_cppflags_save="$CPPFLAGS"
-  CFLAGS="$CFLAGS $X_CFLAGS"
-  CPPFLAGS="$CPPFLAGS $X_CFLAGS"
-
-  dnl now try to find the HEADER
-  HAVE_X="yes"
-  AC_CHECK_HEADER([X11/Xlib.h], [], [HAVE_X="no"], [AC_INCLUDES_DEFAULT])
-  AC_CHECK_HEADER([X11/XKBlib.h], [], [HAVE_X="no"], [AC_INCLUDES_DEFAULT])
-
-  if test "x$HAVE_X" = "xno"
-  then
-    AC_MSG_NOTICE([cannot find X11 development files])
-  else
-    dnl this is much more than we want
-    X_LIBS="$X_LIBS $X_PRE_LIBS $X_EXTRA_LIBS"
-    dnl AC_PATH_XTRA only defines the path needed to find the X libs,
-    dnl it does not add the libs; therefore we add them here
-    X_LIBS="$X_LIBS -lX11"
-    AC_SUBST(X_CFLAGS)
-    AC_SUBST(X_LIBS)
-  fi
-  AC_SUBST(HAVE_X)
-  CFLAGS="$ac_cflags_save"
-  CPPFLAGS="$ac_cppflags_save"
+  AG_GST_CHECK_X
 ])
 
 dnl Check for Xv extension