Check for alternate __unix__ predef.

physfs_platforms.h

@@ -38,7 +38,7 @@
 #  endif
 #elif defined(macintosh)
 #  error Classic Mac OS support was dropped from PhysicsFS 2.0. Move to OS X.
-#elif defined(unix)
+#elif defined(unix) || defined(__unix__)
 #  define PHYSFS_PLATFORM_UNIX
 #  define PHYSFS_PLATFORM_POSIX
 #else