Support building DLLs on Windows. This closes #1627.

Patch by: Vlad Georgescu <vgeorgescu@gmail.com>
          rey4

* build.conf:
  (libsvn_client, libsvn_delta, libsvn_diff, libsvn_fs,
   libsvn_ra, libsvn_repos, libsvn_subr, libsvn_wc):
  Replace the msvc_static option with a msvc_export option containing the
  list of public header files for that library.

* build/generator/extractor.py:
  Handle svn_ctype_table.
  (_filter_names): Add svn_auth_get_keychain_simple_provider.

* build/generator/gen_win.py
  (create_dll_target): New. Given a static library, it creates a new
   dynamic library target that depends on that library.
  (get_install_targets): Call create_dll_target for each target that has
   msvc_export set.
  (get_linked_win_depends): Scan the dependency tree in breadth-first
   order, allowing us to prevent cases when both a dll and its
   corresponding static library are linked into a project.

* build/generator/vcnet_vcproj.ezt:
  Add empty.c as a source file for dll projects, because Visual Studio
  refuses to build a project's outputs if it has no source files.

* build/win32/empty.c: New file.

* gen-make.py:
  Handle the new --disable-shared option.

* subversion/libsvn_ra/ra_loader.c
  (svn_ra_dav_init, svn_ra_svn_init, svn_ra_local_init, svn_ra_serf_init):
  Implement these here if their corresponding libraries are not linked in,
  and have them return a "not implemented" error.

* subversion/libsvn_subr/ctype.c
  (svn_ctype_table_internal): Rename from ctype_table. Remove 'static'
   qualifier.

* win-tests.py:
  Append the paths to our libraries' dlls to the PATH environment variable.