common: Work around vala srcdir != builddir from git issues

gnome-continuous uses srcdir != builddir *from git*.  This differs
from tarballs where generated source lives in $(srcdir).

Anyways, vala's AM_PROG_VALAC generates some makefile rules that do
"cd $(srcdir) && valac ...".

See https://bugzilla.gnome.org/show_bug.cgi?id=694153

The problem is this actively breaks the $(srcdir) and $(builddir)
automake macros when doing srcdir != builddir from git.

We can work around it this way.

https://bugzilla.gnome.org/show_bug.cgi?id=712538