Reported at https://gcc.gnu.org/PR79771: After the import of zlib-1.2.11 in GCC trunk, the zlib in-tree build for cygwin64 is broken under Windows 7 (can't check this myself).

A possible patch / work around can be found at
https://github.com/Alexpux/MSYS2-packages/blob/master/zlib/1.2.11-cygwin-no-widechar.patch