against the new DEF file scripts/symbols.def.
Changed the default DEF file back to pngwin.def.
Removed makefile.mingw.

  and callbacks.
Changed rules for generation of the various symbol files and added a new
  rule for a DEF file (which is also added to the distribution).
Updated the symbol file generation to stop it adding spurious spaces
  to EOL (coming from preprocessor macro expansion).  Added a facility
  to join tokens in the output and rewrite *.dfn to use this.

and Makefile.in to be consistent with changes in libpng-1.4.0rc03