testsuite: Allow setting gpp_std_list in configuration files

This allows the gpp_std_list variable to be set in ~/.dejagnurc instead
of using the GXX_TESTSUITE_STDS environment variable.  This is
consistent with how other defaults such as tool_timeout can be set.

The environment variable can still be used to override the default.

gcc/testsuite/ChangeLog:

	* lib/g++-dg.exp: Update comments.
	* lib/g++.exp (gpp_std_list): Check for an existing value before
	setting it to an empty list.