Fix build for nvc.sim.

This is additional commit for issue #41.
NVC.SIM is now NVC.ENV.

This is additional commit for issue #41.
Build STD.ENV if --enable-vhdl-2008 is specified.

This is additional commit for issue #41.

Last thing I should implement is skipping unsupported regressions
in test/regress/testlist.txt.
Merge remote-tracking branch 'hiyuh/issue41'
Regenerate dependency files
Disable stop2 test if not built with --enable-vhdl-2008