configure.ac: Add subdir-objects to the automake flags

The unit tests depend on files in another directory, and automake 1.14+
will complain when subdir-objects is not set and such a setup is used.
Combined with -Werror, this leads to autoreconf -i failing with a recent
automake.

Since subdir-objects have no ill side-effects, this patch adds that to
the AM_INIT_AUTOMAKE list of options.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Use a convenience library to help the tests

Instead of compiling Edgar twice, once for the REPL, once for the tests,
create a convenience (static) library called libedgar, and link both the
REPL and the test suite to it. This also makes it possible to build
edgar in a subdirectory (like during distcheck) and still have
subdir-objects enabled.

The downside is that this adds a dependency on ar, but that should be
available even on mingw32, I believe.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>