add install.runc target

We need this for platforms like Ubuntu and Debian that do not ship a
standard (post 1.0rc4) version of runc.

I'm assuming here this is why we're building `runc` on our own here -
but it doesn't make sense to just do that while leaving only a symlink
in $PWD. We want to actually install the thing as well. So we add an
`install.runc` target, similar to `install.libseccomp.sudo` to make
sure we install `runc` in the right location.

An alternative to this would be to change the documentation to do the
`install` command by hand, but this is more error-prone. As runc
trickles down to the Debian distros, we can then just remove the `make
install.runc` call and, eventually, the target itself.

Closes: #355

Signed-off-by: Antoine Beaupré <anarcat@debian.org>

Closes: #354
Approved by: rhatdan

Makefile

@@ -78,6 +78,10 @@ install:
 install.completions:
 	install -m 644 -D contrib/completions/bash/buildah $(DESTDIR)/${BASHINSTALLDIR}/buildah
 
+.PHONY: install.runc
+install.runc:
+	install -m 755 ../../opencontainers/runc/runc $(DESTDIR)/$(BINDIR)/
+
 .PHONY: test-integration
 test-integration:
 	cd tests; ./test_runner.sh

install.md

@@ -100,7 +100,7 @@ Then to install Buildah on Ubuntu follow the steps in this example:
   git clone https://github.com/projectatomic/buildah ./src/github.com/projectatomic/buildah
   cd ./src/github.com/projectatomic/buildah
   PATH=/usr/lib/go-1.8/bin:$PATH make runc all TAGS="apparmor seccomp"
-  sudo make install
+  sudo make install install.runc
   buildah --help
 ```