Luca Bacci authored May 03, 2019 and Mike Fleetwood committed May 15, 2019

With the same case as from the previous commit, the very long "Mounted
on ..." text is now wrapped, but the text may not be left justified.
Slowly adjust the dialog width and see how the text wrapping is updated
to fit the size adjustment but the text is centred rather than left
justified.

This is because setting the halign property to Gtk::ALIGN_START does not
guarantee left alignment of text for wrapped or ellipsized Gtk::Labels.

Use the xalign property instead.

To set the xalign property there is a method in the GtkMisc (Gtk::Misc)
base class:

  gtk_misc_set_alignment (Gtk::Misc::set_alignment)

However, GtkMisc (Gtk::Misc) was deprecated in Gtk 3.14 (Gtkmm 3.14)
and in Gtk 3.16 (gtkmm 3.16) set_alignment() was replaced with the
introduction of two new methods:

  gtk_label_set_xalign (Gtk::Label::set_xalign)
  gtk_label_set_yalign (Gtk::Label::set_yalign)

Add a check for Gtkmm method Gtk::Label::set_xalign() in configure.ac
and use it when available.
...