lib/upnp/Compat: disable the 1.8 API emulation with libupnp 1.6.24

libupnp 1.6.24 added a few badly designed macros which break the MPD
build:

 8177a4195a/

To work around this, we disable our emulation functions (from
714011c81ed6053f7a35812071499356f337dd70) on this libupnp version.

Closes #163

NEWS

@@ -1,6 +1,7 @@
 ver 0.20.13 (not yet released)
 * database
   - simple: don't purge mount points on update/rescan
+  - upnp: work around libupnp 1.6.24 API breakage
 
 ver 0.20.12 (2017/11/25)
 * database

src/lib/upnp/Compat.hxx

@@ -23,12 +23,15 @@
 #include <upnp/upnp.h>
 
 #if UPNP_VERSION < 10800
-#include "Compiler.h"
-
 /* emulate the libupnp 1.8 API with older versions */
 
 using UpnpDiscovery = Upnp_Discovery;
 
+#endif
+
+#if UPNP_VERSION < 10624
+#include "Compiler.h"
+
 gcc_pure
 static inline int
 UpnpDiscovery_get_Expires(const UpnpDiscovery *disco) noexcept