Bug#836707: libgtk-3-dev versioned build-dep is too weak

[James Clarke]

Source: clutter-gtk
Version: 1.8.0-2
Severity: important

(Not sure if this counts as RC-critical or not, given it's not really an
FTBFS)

Since clutter-gtk 1.8.0-2 added git_disable-dubbel-buffer.patch it
bumped up gtk_req_version to 3.21.0, but the build dependency for
libgtk-3-dev is still only (>= 3.19.4). This causes it to FTBFS on
architectures which don't yet have gtk+3.0 3.21.0 built (either due to
slow buildds or failed builds), such as sparc64.

Regards,
James