diff --git a/src/gzip/README.freetype b/src/gzip/README.freetype
index 493b80719..e0c8ced18 100644
--- a/src/gzip/README.freetype
+++ b/src/gzip/README.freetype
@@ -18,5 +18,6 @@ The files in this directory have been prepared as follows.
 
  - Take the unmodified source code files from the zlib distribution that are
    included by `ftgzip.c`.
+ - Copy `zconf.h` to `ftzconf.h` (which stays unmodified otherwise).
  - Run zlib's `zlib2ansi` script on all `.c` files.
  - Apply the diff file(s) in the `patches` folder.
diff --git a/src/gzip/ftzconf.h b/src/gzip/ftzconf.h
index 5e1d68a00..bf977d3e7 100644
--- a/src/gzip/ftzconf.h
+++ b/src/gzip/ftzconf.h
@@ -38,6 +38,9 @@
 #  define crc32                 z_crc32
 #  define crc32_combine         z_crc32_combine
 #  define crc32_combine64       z_crc32_combine64
+#  define crc32_combine_gen     z_crc32_combine_gen
+#  define crc32_combine_gen64   z_crc32_combine_gen64
+#  define crc32_combine_op      z_crc32_combine_op
 #  define crc32_z               z_crc32_z
 #  define deflate               z_deflate
 #  define deflateBound          z_deflateBound
@@ -349,6 +352,9 @@
 #    ifdef FAR
 #      undef FAR
 #    endif
+#    ifndef WIN32_LEAN_AND_MEAN
+#      define WIN32_LEAN_AND_MEAN
+#    endif
 #    include <windows.h>
      /* No need for _export, use ZLIB.DEF instead. */
      /* For complete Windows compatibility, use WINAPI, not __stdcall. */
@@ -467,11 +473,18 @@ typedef uLong FAR uLongf;
 #  undef _LARGEFILE64_SOURCE
 #endif
 
-#if defined(__WATCOMC__) && !defined(Z_HAVE_UNISTD_H)
-#  define Z_HAVE_UNISTD_H
+#ifndef Z_HAVE_UNISTD_H
+#  ifdef __WATCOMC__
+#    define Z_HAVE_UNISTD_H
+#  endif
+#endif
+#ifndef Z_HAVE_UNISTD_H
+#  if defined(_LARGEFILE64_SOURCE) && !defined(_WIN32)
+#    define Z_HAVE_UNISTD_H
+#  endif
 #endif
 #ifndef Z_SOLO
-#  if defined(Z_HAVE_UNISTD_H) || defined(_LARGEFILE64_SOURCE)
+#  if defined(Z_HAVE_UNISTD_H)
 #    include <unistd.h>         /* for SEEK_*, off_t, and _LFS64_LARGEFILE */
 #    ifdef VMS
 #      include <unixio.h>       /* for off_t */