We can't really use `dlopen()` from a statically built binary, so
let's disable this functionality if `static_build` tag is used
(which is sort of a de-facto standard way of doing it).
This eliminates the following warning:
🇩 bin/containerd
# github.com/containerd/containerd/cmd/containerd
/tmp/go-link-509179974/000004.o: In function `pluginOpen':
/usr/local/go/src/plugin/plugin_dlopen.go:19: warning: Using 'dlopen' in
statically linked applications requires at runtime the shared libraries
from the glibc version used for linking
[v2: add static build instructions to BUILDING.md]
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
