buildroot

vala: add vala/valac wrapper

vala/valac can use gir and vapi data files installed by other packages,
but since these are normally installed to staging and host-vala looks
for them in the host directory (logically) this leads to failure.
So wrap them to call the real tool and add this information via
command-line parameters to them.

This is required for vala-in-vala bindings (vapi).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>