makedep: Check for external libraries instead of include paths when determining whether to allow external includes.

The include path may be empty, e.g. if the relevant headers are to be found
directly in the MinGW sysroot.
Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>