bump glib (and gio, gthread, etc) minimum to 2.50.0

In order to unify dependencies for core components.
(See 4.16 roadmap)

configure.ac.in

@@ -147,10 +147,10 @@ XDT_CHECK_PACKAGE([LIBXFCE4UI], [libxfce4ui-2], [4.15.1])
 XDT_CHECK_PACKAGE([XFCONF], [libxfconf-0], [4.13.2])
 XDT_CHECK_PACKAGE([GTK], [gtk+-3.0], [3.22.0])
 XDT_CHECK_PACKAGE([EXO], [exo-2], [0.11.2])
-XDT_CHECK_PACKAGE([GLIB], [glib-2.0], [2.42.0])
-XDT_CHECK_PACKAGE([GIO], [gio-2.0], [2.42.0])
-PKG_CHECK_MODULES([GIO_UNIX], [gio-unix-2.0 >= 2.42.0])
-XDT_CHECK_PACKAGE([GMODULE], [gmodule-2.0], [2.42.0])
+XDT_CHECK_PACKAGE([GLIB], [glib-2.0], [2.50.0])
+XDT_CHECK_PACKAGE([GIO], [gio-2.0], [2.50.0])
+PKG_CHECK_MODULES([GIO_UNIX], [gio-unix-2.0 >= 2.50.0])
+XDT_CHECK_PACKAGE([GMODULE], [gmodule-2.0], [2.50.0])
 XDT_CHECK_PACKAGE([CAIRO], [cairo], [1.0.0])
 XDT_CHECK_PACKAGE([LIBWNCK], [libwnck-3.0], [3.0])
 
@@ -163,7 +163,7 @@ dnl **********************************
 dnl *** Optional GIO UNIX features ***
 dnl **********************************
 XDT_CHECK_OPTIONAL_PACKAGE([GIO_UNIX], [gio-unix-2.0],
-                           [2.42.0], [gio-unix], [GIO UNIX features])
+                           [2.50.0], [gio-unix], [GIO UNIX features])
 
 dnl ***************************************
 dnl *** Check for gobject-introspection ***