Define gdouble endianness conversion macros

GLib doesn’t have these for some reason. These are protected in case a
later version of GLib defines them, too.

src/astrognome.h

@@ -15,5 +15,41 @@ extern GtkFileFilter *filter_chart;
 const gchar *ag_house_system_id_to_nick(GsweHouseSystem house_system);
 GsweHouseSystem ag_house_system_nick_to_id(const gchar *nick);
 
+#ifndef GDOUBLE_FROM_LE
+inline static gdouble
+GDOUBLE_SWAP_LE_BE(gdouble in)
+{
+    union {
+        guint64 i;
+        gdouble d;
+    } u;
+
+    u.d = in;
+    u.i = GUINT64_SWAP_LE_BE(u.i);
+
+    return u.d;
+}
+
+#if (G_BYTE_ORDER == G_LITTLE_ENDIAN)
+
+#define GDOUBLE_TO_LE(val)   ((gdouble)(val))
+#define GDOUBLE_TO_BE(val)   (GDOUBLE_SWAP_LE_BE(val))
+
+#elif (G_BYTE_ORDER == G_BIG_ENDIAN)
+
+#define GDOUBLE_TO_LE(val)   (GDOUBLE_SWAP_LE_BE (val))
+#define GDOUBLE_TO_BE(val)   ((gdouble) (val))
+
+#else
+
+#error "unknown endian type"
+
+#endif
+
+#define GDOUBLE_FROM_LE(val) (GDOUBLE_TO_LE (val))
+#define GDOUBLE_FROM_BE(val) (GDOUBLE_TO_BE (val))
+
+#endif /* !defined GDOUBLE_FROM_LE */
+
 #endif /* __ASTROGNOME_H__ */