The "_DLL" is removed from the test of whether a DLL is being built.

Also corrected zlib.vcxproj into which Visual Studio had introduced
what it calls an "authoring error".  The change to make pnglibconf.h
simply copies the file; in the future it may actually generate the
file from scripts/pnglibconf.dfa as the other build systems do.

  The changes include support for function attributes in VC in addition to
    those already present in GCC - necessary because without these some
    warnings are unavoidable.  Fixes include signed/unsigned fixes in
    pngvalid and checks with gcc -Wall -Wextra -Wunused.
  VC requires function attributes on function definitions as well as
    declarations, PNG_FUNCTION has been added to enable this and the
    relevant function definitions changed.

that were recently introduced, mostly resulting from
safe but uncast assignments to shorter integers.  Also added a zlib
VStudio release library project because the latest zlib Official Windows
build does not include such a thing.