Let's warn about binary needed for `install` target.
Before:
[root@localhost criu]# make install
  GEN      crit.1
make[1]: *** [Makefile:49: crit.1] Error 127
make: *** [Makefile.install:32: install-man] Error 2

After:
[root@localhost criu]# make install
/bin/sh: line 1: xmlto: command not found
make[1]: *** [Makefile:32: check] Error 1
make: *** [Makefile.install:32: install-man] Error 2

Fixes: #481
Reported-by: treaki@gmail.com
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>