Post-release version bump to 1.12
build-sys: Fix parallel build

On my machine, parallel builds fail with:

make[2]: Entering directory `/home/teuf/hack/libplist/src'
  CCLD     libplist.la
make[2]: *** No rule to make target `../src/libplist.la', needed by
`libplist++.la'.  Stop.

If $(top_builddir)/src/libplist.la does not exist yet when trying to
link libplist++.la, automake/make will not realize the
$(top_builddir)/src/libplist.la dependency is the same as the
libplist.la target, and will thus be unable to generate
$(top_builddir)/src/libplist.la.

Using the libplist.la instead fixes this issue.

I've checked that srcdir!=builddir and make distcheck still pass after
this change.