Use RPATH for lib folder, use PHYSFS_getBaseDir on all platforms.

The next official VVVVVV build removes 32-bit Linux (like all my other games),
and I need to get rid of the shell script on macOS at some point, so this
basically brings it up to what my other games are doing. Plus, this probably
fixes a bug where someone tries to run their executable away from the root...

desktop_version/src/FileSystemUtils.cpp

@@ -70,12 +70,8 @@ int FILESYSTEM_init(char *argvZero)
 	}
 
 	/* Mount the stock content last */
-#ifdef _WIN32
 	strcpy(output, PHYSFS_getBaseDir());
 	strcat(output, "data.zip");
-#else
-	strcpy(output, "data.zip");
-#endif
 	if (!PHYSFS_mount(output, NULL, 1))
 	{
 		puts("Error: data.zip missing!");