[Vala] vala cross-compiling

[Michele Dionisio <michele dionisio gmail com>]

It is not possible any more to cross-compile vala after commit

797a15747b7c0cb43ea7e13687f1af34f6e14ddf     build: Avoid using ".." for
referencing paths and reorganize valac-calls
7a4f0befbd58286a69367ba78d16187da25724f7    build: Make building vapigen
non-optional

because the builded vala is used during compilation. But If I'm
crosscomplling I cannot use the builded vala because I'n not on the target
machine.

Rollingback the 2 checkin everythings works well also on latest master.

regards.