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

This closes #461 issue on GitHub.

This is required for futher ASGI implementation.

No functional changes.  Get ready for an increase in file number.

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.

Python 3.8 has 'tp_print' field in PyTypeObject struct.  This field is
attributed as deprecated.  So, clang generates warning (which is turned to
error) as a result of initializing this field.  From the other hand, it is
impossible to omit this field in positional initialization.  The solution
is to use designated initializer.

Silencing usage message during configure python.

This is related to #331 issue on GitHub.