Add CPPFLAGS to "shared if possible" test binaries

Add CPPFLAGS to "shared if possible" test binaries

In principle we ought to define DBUS_STATIC_BUILD in anything that's
using libdbus-internal.la (to avoid linking failures on
statically-linked mingw builds), and DBUS_TEST_USE_INTERNAL in any
test that's using the non-dbus-glib code paths of test-utils.[ch]
(to avoid the GLib requirement, although in practice, everything
"shared if possible" requires GLib at the moment anyway).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=68852
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>