build-sys: Fix relinking of _journal.la for parallel builds

Automake assumes that all LTLIBRARIES can be installed in parallel.
However, libtool relinks the libraries during install, so all needed
libraries must be installed before the relinking starts.

This is a problem for the _journal.la python library, which must be
relinked against the installed libsystemd-journal.la. With parallel make
jobs, a race condition is introduced since nothing forces
libsystemd-journal.la to be installed before the install rules for
_journal.la start.

Add a terrible hack to force installation of the python libraries to
depend on completion of the core library installation. This requires
usage of the automake internal make targets and indirection through a
make variable so that automake will keep the original
install-pkgpyexecLTLIBRARIES recipe.