indicator fails to build with newest vala + glib

[Fabio Valentini]

The latest betas of some GNOME libraries (glib2, vala, etc.) break the wingpanel-indicator-notifications build. The following errors occur during build:

> /builddir/build/BUILD/wingpanel-indicator-notifications-2.0/src/Widgets/NotificationEntry.vala:113.92-113.95: error: Argument 1: Cannot convert from `ulong' to `long'
> return dngettext (Config.GETTEXT_PACKAGE, "%ld day", "%ld days", days).printf (days);
> ^^^^
> /builddir/build/BUILD/wingpanel-indicator-notifications-2.0/src/Widgets/NotificationEntry.vala:116.95-116.99: error: Argument 1: Cannot convert from `ulong' to `long'
> return dngettext (Config.GETTEXT_PACKAGE, "%ld hour", "%ld hours", hours).printf (hours);
> ^^^^^
> /builddir/build/BUILD/wingpanel-indicator-notifications-2.0/src/Widgets/NotificationEntry.vala:119.101-119.107: error: Argument 1: Cannot convert from `ulong' to `long'
> return dngettext (Config.GETTEXT_PACKAGE, "%ld minute", "%ld minutes", minutes).printf (minutes);
> ^^^^^^^
> make[2]: *** [src/CMakeFiles/notifications-indicator.dir/build.make:112: src/notifications-indicator_valac.stamp] Error 1

The relevant version changes are (I think):

vala 0.34.4-1.fc26 > 0.35.3-1.fc26
glib2 2.51.0-2.fc26 > 2.51.1-1.fc26

A list of all build dependencies that changed between the last good and the first broken build can be found at: https://apps.fedoraproject.org/koschei/build/2578284

This breakage might indicate a future problem for this package on elementaryOS loki+1(?), too, depending on which version of GNOME libraries make it into ubuntu in time (...)