build: explicitely requires standard math library.
gv_interface: fix crash when non INET interface available.

Fix https://bugzilla.gnome.org/show_bug.cgi?id=710606
viewer: check for gstreamer plugins at application launch.
build: explicitely requires zlib.
build: make configure work even without GOBJECT_INTROSPECTION_CHECK.
build: remove GOBJECT_INTROSPECTION_CHECK leftover.
camera: fix a possible NULL dereference crash.

It would happen if either minimum or maximum placeholder is NULL, and
using a The Imagning Source camera.
evaluator: fix comparisons when one of the operand is a double.

fixes https://bugzilla.gnome.org/show_bug.cgi?id=720809.