I rebased, this fix is currently still valid and needed. Sadly it seems that pull requests here get ignored. I'll try to reach the correct contribution channel e.g. mailing list.

@julianoes I wrote a mail to the developers:

Dear zlib authors and/or developers

I hope this is the appropriate way to suggest changes. If not please forgive me and forward this mail to any necessary list or developer.

I'm PX4 drone autopilot developer and the related DronecodeSDK to monitor and control drones depends on zlib. As Windows user I'm maintaining cygwin support for PX4.

PX4 https://github.com/PX4/Firmware
DronecodeSDK https://github.com/Dronecode/DronecodeSDK
Cygwin https://www.cygwin.com/

Now on cygwin there are two issues with the zlib usage:

• zlib build error related to fwopen on master (cacf7f1) which is fixed on develop already ✔️:

/cygdrive/c/PX4/home/DronecodeSDK/third_party/zlib/gzlib.c: In function ‘gz_open’:
/cygdrive/c/PX4/home/DronecodeSDK/third_party/zlib/gzlib.c:243:20: warning: implicit declaration of function ‘_wopen’; did you mean ‘fwopen’? [-Wimplicit-function-declaration]
         fd == -2 ? _wopen(path, oflag, 0666) :
                    ^~~~~~
                    fwopen

• zlib build error related to windres on develop (41d86c7) only with cmake build ❌:

/usr/bin/windres: /cygdrive/c/PX4/home/DronecodeSDK/third_party/zlib/win32/zlib1.rc:7: syntax error
make[2]: *** [CMakeFiles/zlib.dir/build.make:423: CMakeFiles/zlib.dir/win32/zlib1.obj] Error 1
make[1]: *** [CMakeFiles/Makefile2:105: CMakeFiles/zlib.dir/all] Error 2
make: *** [Makefile:139: all] Error 2

Please consider applying the following patch on top of develop (41d86c7) to solve the second problem:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e108c16..fa073c7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -167,7 +167,7 @@ file(READ ${CMAKE_CURRENT_SOURCE_DIR}/zlib.h _zlib_h_contents)
 string(REGEX REPLACE ".*#define[ \t]+ZLIB_VERSION[ \t]+\"([-0-9A-Za-z.]+)\".*"
     "\\1" ZLIB_FULL_VERSION ${_zlib_h_contents})

-if(MINGW)
+if(MINGW OR CYGWIN)
     # This gets us DLL resource information when compiling on MinGW.
     if(NOT CMAKE_RC_COMPILER)
         set(CMAKE_RC_COMPILER windres.exe)

My suggestion is also available as pull request on GitHub for convenient merging: #367

Thanks a lot in advance!

Best regards
Matthias Grob (alias MaEtUgR)

I'm closing this since there seems to be no maintenance going on since 2017. Pity, I hoped the developer would come back or hand it over to another maintainer.

It looks like there is a maintained fork by chromium but not on GitHub unfortunately.
https://chromium.googlesource.com/chromium/src/third_party/zlib

@MaEtUgR: Any news with Zlib 1.3.1?