steam-util: added g_strcmp0 to keep compatible with glib < 2.16

As of bitlbee 3.2, the required version of glib is only 2.14. This must
be the version that the plugin depends on.

A dependency was made on glib >= 2.16 by the usage of g_strcmp0. To keep
compatible, define g_strcmp0 if it is missing.
steam-http: compare headers and parameters while ignoring their casing

This prevents doubly storing headers and parameters that have the same
name, but different casing.