I tried using valac during cross compilation and figured that one
presently gets an "Exec format error", because it is implicitly marked
Multi-Arch: no. So I considered whether it can be marked Multi-Arch:
foreign and I conclude "no", because valac uses gcc and pkg-config and
exposes their architecture-dependent behaviour. Thus we must consider
valac to be a "tool". That means considering $ac_tool_prefix.
AM_PROG_VALAC currently uses AC_PATH_PROG, so it does not consider
$ac_tool_prefix. I think this is a bug and it really should be using
AC_PATH_TOOL here. Please consider applying the attached patch.

Helmut