glib-networking add environment variable proxy PACKAGECONFIG

Add a PACKAGECONFIG for the environment variable proxy configuration
(http_proxy, etc), and enable by default as this is the fallback
provider if you're not using libproxy or gnomeproxy.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>