Thanks to @wujjpp.

This closes #490 PR on GitHub.

This variant will be more interoperable across various systems
and it's already used in Ruby module.

Otherwise, configure tests fail on NetBSD with:

  gcc: Missing argument for -Wl,-rpath

Now it's possible to disable default bind mounts of
languages by setting:

  {
      "isolation": {
           "automount": {
               "language_deps": false
           }
     }
  }

In this case, the user is responsible to provide a "rootfs"
containing the language libraries and required files for
the application.

Since the introduction of rootfs feature, some language modules
can't be configured multiple times.

Now the configure generates a separate nxt_<module>_mounts.h for
each module compiled.

Now the configure script appends /server to --lib-path argument.