Build subversion libraries as dlls. This change has no effect on the
generated makefile.

* build.conf
  (libsvn_client, libsvn_diff, libsvn_fs, libsvn_ra, libsvn_repos,
   libsvn_subr, libsvn_wc)
    no longer set "msvc-static" option. set "msvc-export" option to
    point to header files to scan for exported functions

* build/generator/gen_base.py
  (TargetLib.__init__):
    read "msvc-export" option

* build/generator/msvc_dsp.ezt
* build/generator/vcnet_vcproj.ezt
    add custom build steps for generating module definition (.def)
    files

* build/generator/gen_win.py
  (WinGeneratorBase.get_proj_sources):
    add in logic for .def custom builds, also switch around
    parameters for swig custom builds

* build/generator/extractor.py
  (_funcs, extract_funcs):
    make regular expression for finding function declarations a
    little more tolerant

  (scan_headers, headers_to_libraries):
    removed. header information is now stored in build.conf

  (global code):
    change to output function list in .def format

* subversion/include/svn_md5.h
* libsvn_subr/md5.c
  (svn_md5_empty_string_digest):
    convert this from an extern global into a function so it can
    be exported
  
* subversion/libsvn_fs/reps-strings.c
  (svn_fs__get_mutable_rep, rep_contents_clear):
    update references to svn_md5_empty_string_digest