remove implicit c:/cygwin, fix logic some more

dtool/src/dtoolutil/filename.cxx

@@ -133,12 +133,9 @@ get_panda_root() {
     const char *envvar = getenv("PANDA_ROOT");
     if (envvar != (const char *)NULL) {
       panda_root = front_to_back_slash(envvar);
-    } else if (Filename("c:/cygwin").is_directory()) {
-      //setenv("PANDA_ROOT", "c:/cygwin", 1);
-      panda_root = "c:/cygwin";
     }
 
-    if (!panda_root.empty() || panda_root[panda_root.length() - 1] != '\\') {
+    if (!panda_root.empty() && panda_root[panda_root.length() - 1] != '\\') {
       panda_root += '\\';
     }