configure.in: revert part of c5b3ab002c50f5b23391d7365966e3dc5561306f

The change to fix the detection of libSDL_image*.so for Debian does not
work on 64-bit Debian 7.

Revert the change to the old style.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Makefile.am fixes $(SIGC_LIBS)

The $(SIGC_LIBS) need to be included in the link flags for some
of the test applications to link correctly.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>