net-mgmt/icinga2:

Remove bogus dependency on devel/libconfig. Some time ago I added this because
the ports tree's Q/A check warned that the icinga2 binary is linked against libconfig.
However this is a private library shipped with Icinga 2.
With version 2.5.1 USES=ssl was added to the port's Makefile which added
rpath=${LOCALBASE}/openssl when Icinga 2 gets built and {Open,Libre}SSL from ports
was installed. The result was that ld picked up libconfig.so from the devel/libconfig
port instead of Icinga 2's private library.
To prevent this, the following CMAKE_ARGS were added:
CMAKE_{EXE,MODULE,SHARED,STATIC}_LINKER_FLAGS=${PREFIX}/lib/icinga2

At the moment only EXE_ and SHARED_ linker flags are really needed, to be prepared for
future changes the other flags were also set.

Thanks to mat and Gunnar Beutner (upstream developer) for the insights on the ports
and Icinga 2's build system.

• Re-add USES=ssl
• Bump PORTREVSION