Much nicer handling of compile-time features

- *_SUPPORT enabled in config.h
- XLIBS helper app prints lib flags needed for current settings

config.def.h

@@ -1,3 +1,15 @@
+#ifdef _FEATURE_CONFIG
+
+/* auto-orientate jpeg files according to their exif tags?
+ * (requires libexif [-lexif] to be installed)
+ */
+#define EXIF_SUPPORT 0
+/* load all frames from gif files and support gif animations?
+ * (requires giflib [-lgif] to be installed)
+ */
+#define GIF_SUPPORT  0
+
+#endif
 #ifdef _WINDOW_CONFIG
 
 /* default window dimensions (overwritten via -g option): */