decoder/modplug: assume ModPlug is built as static library on Windows

NEWS

@@ -4,6 +4,7 @@ ver 0.21.24 (not yet released)
 * database
   - simple: fix crash when mounting twice
 * decoder
+  - modplug: fix Windows build failure
   - wildmidi: attempt to detect WildMidi using pkg-config
   - wildmidi: fix Windows build failure
 * Android

src/decoder/plugins/ModplugDecoderPlugin.cxx

@@ -26,8 +26,13 @@
 #include "util/RuntimeError.hxx"
 #include "Log.hxx"
 
-#include <libmodplug/modplug.h>
+#ifdef _WIN32
+/* assume ModPlug is built as static library on Windows; without
+   this, linking to the static library would fail */
+#define MODPLUG_STATIC
+#endif
 
+#include <libmodplug/modplug.h>
 
 #include <assert.h>