By default, pkg-config-rs sets the PKG_CONFIG_ALLOW_SYSTEM_LIBS
environment variable
(https://github.com/alexcrichton/pkg-config-rs/pull/35), which leads
to eg /usr/lib being present in the middle of the final command,
in turn leading to issues when working in an uninstalled environment,
where libraries that are also present system-wide do not get linked
against.
gst_flow_combiner_ref/unref were introduced in gstreamer-1.12.1 but they
inherit the `since version` declared for `GstFlowCombiner` which is 1.4.
Fixes https://github.com/sdroege/gstreamer-sys/pull/5
