When building into a custom prefix, the icons in PREFIX/share/icons/ may
not be found in some cases, thus, when building also with
the ENABLE_MAINTAINER_MODE, add also that directory into the gtk+
icon theme search path.
Let's drop dependency on something more or less unmaintained, when
there's used only a little subpart of it, which can be extracted
and used directly in Evolution. This particular part didn't change
in libcryptui for more than four years.
libical supports ICAL_FILENAME_PARAMETER since 2.0.0. It's part of
an extension which sets filename for attachments, thus even inline
attachments keep their original filename.
