build: support explicitly disabling netcf

build: support explicitly disabling netcf

placing "-Dnetcf=disabled" on the meson commandline was ignored,
meaning that even with that option the build would get WITH_NETCF if
the netcf-devel package was found - the only way to disable it was to
uninstall netcf-devel.

This patch adds the small bit of logic to check the netcf meson
commandline option (in addition to whether netcf-devel is installed)
before defining WITH_NETCF.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>