Makefile: allow overriding install command via environment
Makefile: allow overriding install command via environment

Packaging systems often want to override the install(1) command via INSTALL
environment variable, in order to do distro specific fixups (eg. splitting
out debug symbols from binaries to separate files).

Also use it for creating install target directories.

Signed-off-by: Enrico Weigelt, metux IT consult <59bd0a3ff43b32849b319e645d4798d8a5d1e889@metux.net>

thaJeztah/containerd,thaJeztah/containerd,dmcgowan/containerd,vdemeester/containerd,mikebrow/containerd,vdemeester/containerd,containerd/containerd,dmcgowan/containerd,estesp/containerd,mikebrow/containerd,containerd/containerd,estesp/containerd