mate-base/mate-panel: fix compilation failure with networkmanager #570266

mate-base/mate-panel: fix compilation failure with networkmanager #570266

mate-panel-1.8.1 fails to compile with >=net-misc/networkmanager-1.0.8
due to a missing include directive ('-I'). Substitute network manager
CFLAGS and LIBS supplied by the pkgconfig macro in configure.ac and
use it for applets/clock.

Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=570266
Upstream-PR-URL: https://github.com/mate-desktop/mate-panel/pull/388

Package-Manager: portage-2.2.24
Signed-off-by: NP-Hardass <NP-Hardass@gentoo.org>