don't disable use of deprecated APIs unless asked to do so. add Meta case

don't disable use of deprecated APIs unless asked to do so. add Meta case

* configure.in: don't disable use of deprecated APIs unless asked to do so.
* src/keymap.c(_vte_keymap_GDK_space): add Meta case (gafton).
* src/vtefc.c(_vte_fc_width_from_pango_stretch): add.
* src/vtefc.c(_vte_fc_transcribe_from_pango_font_description): handle
  PangoStretch characteristics using _vte_fc_width_from_pango_stretch.
* src/vteregex.c: add random comments.
* src/vte.c(VteTerminal): track the "xn" terminal attribute.