net-im/dino: specify valac for build system to use

net-im/dino: specify valac for build system to use

Before, dino's cmake scripts just searched for unversioned valac, and
versioned ones with versions in the range 0.32 - 0.38.

There are good reasons to let Portage decide which valac to use.

* Now we have 0.40 and 0.42 in Gentoo tree. These are not detected by
  dino build system (bug 678402).
* Allow users to override valac version uniformly across Gentoo with
  VALAC envvar.

Suggested-by: Daniel Kenzelmann <gentoo@k8n.de>
Bug: https://bugs.gentoo.org/678402
Package-Manager: Portage-2.3.51, Repoman-2.3.12
Signed-off-by: Andrey Utkin <andrey_utkin@gentoo.org>