GLib version requirement

I see that since 2017-02-09 gnuplot requires version 2.28 or higher of 
GLib, on account of use of the macro g_clear_object.

Unfortunately it can be difficult to build GLib on (or for) some older 
systems, such as OS X <= 10.4, but older GLib versions are easier 
(e.g. 2.18). I'd like to suggest pushing the GLib dependency back to 
2.18 and adding a simple compatibility macro in gp_cairo.c. Here's an
illustrative diff relative to 5.2.3:

--- gp_cairo.c.orig 2018-05-09 16:46:59.510301745 -0400
+++ gp_cairo.c 2018-05-10 08:32:07.378541918 -0400
@@ -77,6 +77,13 @@
  #define rint(x) floor((x)+0.5L)
  #endif

+#if GLIB_MAJOR_VERSION == 2 && GLIB_MINOR_VERSION < 28
+#define g_clear_object(op) \
+    { gpointer *_p = (gpointer) (op); \
+    if (_p && *_p) { g_object_unref(*_p); *_p = NULL; } \
+    }
+#endif
+
  /* undef this to see what happens without the Symbol-to-unicode processing */
  #define MAP_SYMBOL

Any thoughts?

-- 
Allin Cottrell
Department of Economics
Wake Forest University, NC