Fix build with zlib on MS-Windows.

Fix build with zlib on MS-Windows.

 configure.ac (LIBZ): Comment on w32 peculiarities regarding LIBZ.

 src/decompress.c [WINDOWSNT]: Include windows.h and w32.h.
 (DEF_ZLIB_FN, LOAD_ZLIB_FN) [WINDOWSNT]: New macros.  Use them to
 define static variables that are pointers to zlib functions to be
 dynamically loaded.
 (init_zlib_functions) [WINDOWSNT]: New function.
 (fn_inflateInit2_, fn_inflate, fn_inflateEnd, fn_inflateInit2):
 New macros.
 (Fdecompress_gzipped_region, unwind_decompress): Use the fn_*
 macros instead of invoking the zlib functions directly.
 (syms_of_decompress): DEFSYM Qzlib_dll.  Staticpro
 Szlib_available_p.

 lisp/term/w32-win.el (dynamic-library-alist): Add DLLs for zlib.