Log Message:
glib2-tools: Handle source files which are not utf-8

python 3 is strict about file encodings. After glib-mkenums was
translated from perl to python, build failures occur due to e.g.,
ISO-8859-1 characters in authors' names as reported by martin@ in
PR pkg/52631 for xfce4-gtk2-engines, but also causing build failures
for at least libgnome and libgnomeui.

Chritoph Reiter produced an elegant workaround, making use of the
open() function's error decoding "replace" option:

  'replace' causes a replacement marker (such as '?') to be inserted
  where there is malformed data.

which is applied here.

https://bugzilla.gnome.org/show_bug.cgi?id=785113#c27