Make our build system support a disable dirauth module option. It can only be
disabled explicitly with:

  $ ./configure --disable-module-dirauth

If *not* specified that is enabled, an automake conditional variable is set to
true and a defined value for the C code:

  AM_CONDITIONAL: BUILD_MODULE_DIRAUTH
  AC_DEFINE: HAVE_MODULE_DIRAUTH=1

This introduces the dirauth/ module directory in src/or/ for which .c files
are only compiled if the BUILD_MODULE_DIRAUTH is set.

All the header files are compiled in regardless of the support so we can use
the alternative entry point functions of the dirauth subsystem.

Signed-off-by: David Goulet <dgoulet@torproject.org>