Conditionally use font size unit depending on Gtk version

We managed to not do these kind of things for a long time, but here
we go - older versions don't allow it and new versions require it.

lib/howl/ui/theme.moon

@@ -75,13 +75,18 @@ parse_color = (spec, alpha = 1) ->
   tostring(c)
 
 parse_font = (font = {}) ->
+  font_size_unit = 'px'
+
+  if Gtk.get_major_version! == 3 and Gtk.get_minor_version! < 10
+    font_size_unit = ''
+
   size = config.font_size
   decls = {
     "font-family: #{config.font};"
   }
 
   if size
-    append decls, "font-size: #{config.font_size}px;"
+    append decls, "font-size: #{config.font_size}#{font_size_unit};"
 
   if font.italic
     append decls, "font-style: italic;"

lib/ljglibs/cdefs/gtk.moon

@@ -432,4 +432,8 @@ ffi.cdef [[
   /* Misc */
   gboolean gtk_cairo_should_draw_window (cairo_t *cr,
                                          GdkWindow *window);
+
+  guint gtk_get_major_version (void);
+  guint gtk_get_minor_version (void);
+  guint gtk_get_micro_version (void);
 ]]

lib/ljglibs/gtk/init.moon

@@ -53,4 +53,8 @@ core.auto_loading 'gtk', {
 
   cairo_should_draw_window: (cr, window) ->
     C.gtk_cairo_should_draw_window(cr, window) != 0
+
+  get_major_version: -> tonumber C.gtk_get_major_version!
+  get_minor_version: -> tonumber C.gtk_get_minor_version!
+  get_micro_version: -> tonumber C.gtk_get_micro_version!
 }