configure: add hard dependency on glib-networking.

Use a code test inspired by libsoup configure test.
This is a hard dependency because HTTPS should not be considered an
option anymore. Nowadays most websites will use HTTPS by default, HTTP
gives SEO penalties and browsers are starting to display various
security warnings on HTTP websites.
Also the experience will be significantly degraded without SSL/TLS
support since the help browser will fail to load the manual remotely,
and opening various remote files on secure protocols will fail.
Note: the test cannot be performed while cross-compiling. In this case,
we will just display a warning for packagers to be at least well aware
of this dependency.