libtool needs to relink at install (on some platforms) which requires
dependency libraries to be already installed. automake does not
generate the appropriate dependency so we do it here automatically. As
an additional bonus, we have to trick automake using a variable
indirection for the rule because it would otherwise omit the default
install rule.
This should solve the dependency issue breaking parallel build.
Instead of specifying dependencies on in-project libraries using --pkg,
we now directly specify the corresponding .vapi in sources, which makes
it properly depend.