Codechange: check if a define is set directly, instead of indirectly

config.lib happens to set GLOBAL_DATA_DIR in case it is not DOS
and not OS2, but this kind of deduction is annoying to maintain.
It is better to just check if the define you want to use is set,
and leave it to config.lib to set it or not depending on the OS.

src/fileio.cpp

@@ -1121,7 +1121,7 @@ void DetermineBasePaths(const char *exe)
 		}
 	}
 
-#if defined(DOS) || defined(OS2)
+#if !defined(GLOBAL_DATA_DIR)
 	_searchpaths[SP_INSTALLATION_DIR] = NULL;
 #else
 	seprintf(tmp, lastof(tmp), "%s", GLOBAL_DATA_DIR);