I am trying to compile the latest from scratch under MSYS/MINGW64.

The mingw64 installation has system zlib. The compile fails as this has ZLIB_VERNUM = 0x12d0 wheras FLTK PNG requires 0x12b0.

I have tried to run Cmake with OPTION_USE_SYSTEM_OFF=OFF and it still fails.

I have moved the MINGSW zlib.h out of the way and it fails not being able to find zlib.h as if the include library is still looking in the mingw64 directory rather than in the fltk files.

In my MINGW window I gave run

git clone https://github.com/fltk/fltk.git

mkdir -p build/Debug

cd build/Debug

cmake -G "Unix Makefiles" -D 'CMAKE_BUILD_TYPE=Debug' -DCMAKE_SYSTEM_NAME="Windows" -DOPTION_USE_SYSTEM_ZLIB=OFF -DOPTION_USE_SYSTEM_LIBJPEG=OFF -DOPTION_USE_SYSTEM_LIBPNG=OFF ../..

make

and it fails in make. After compiling fltk_zlib and fltk_jpeg it fails on the first png file. I found the order of the defines to be sigificant - setting zLIB after the other two was ignored and the mingsw zlib found instead.

The fail (using make -d) was:-

[ 11%] Building C object png/CMakeFiles/fltk_png.dir/png.c.obj
Reaping winning child 0x800088250 PID 4159
Live child 0x800088250 (png/CMakeFiles/fltk_png.dir/png.c.obj) PID 4160
In file included from C:/users/pvros/dev/fltk/png/png.c:14:
C:/users/pvros/dev/fltk/png/pngpriv.h:920:4: error: #error ZLIB_VERNUM != PNG_ZLIB_VERNUM "-I (include path) error: see the notes in pngpriv.h"
  920 | #  error ZLIB_VERNUM != PNG_ZLIB_VERNUM \
      |    ^~~~~
Reaping losing child 0x800088250 PID 4160
make[2]: *** [png/CMakeFiles/fltk_png.dir/build.make:77: png/CMakeFiles/fltk_png.dir/png.c.obj] Error 1

Thanks Phil