Fix build with TDM-GCC

@newfrenchy83 said that the build is failing:
37225d24ea (commitcomment-25629052)

It looks like TDM-GCC uses Windows XP development headers, unfortunately.
However, the problem is easy to work around by simply #defining the missing
value ourselves. We should consider dropping TDM-GCC support after the 1.14
release, though.

src/filesystem_boost.cpp

@@ -38,6 +38,10 @@ using boost::uintmax_t;
 #include <windows.h>
 #include <shlobj.h>
 #include <shlwapi.h>
+
+// Work around TDM-GCC not #defining this according to @newfrenchy83.
+#define VOLUME_NAME_NONE 0x4
+
 #endif /* !_WIN32 */
 
 #include "config.hpp"