[gcc] tolerate paths with trailing / in $PATH

See previous commit.

component/defaults.cfg

@@ -61,7 +61,7 @@ init =
     for path in os.getenv('PATH', '').split(os.pathsep): # PY3: shutil.which
       gcc = os.path.join(path, 'gcc')
       if os.access(gcc, os.X_OK) and not os.path.isdir(gcc):
-        options['prefix'] = os.path.dirname(path)
+        options['prefix'] = os.path.dirname(os.path.normpath(path))
         break
   else:
     options['prefix'] = self.buildout[options['part']]['location']