I tried to build version 6.8.0 under CYGWIN and I got this error at compile time:

qtbase-everywhere-src-6.8.0/src/corelib/io/../../3rdparty/forkfd/forkfd.c:253:20: error: 'WEXITED' undeclared (first use in this function); did you mean 'WIFEXITED'?
  253 |     int woptions = WEXITED;
      |                    ^~~~~~~
      |                    WIFEXITED

It looks like there are some tests about the presence of WEXITED into the code:

https://github.com/qt/qtbase/blob/eb31aa9cf3d902cc56bc943bf1876cdbf9878298/src/3rdparty/forkfd/forkfd.c#L71

Several functions have an #ifdef HAVE_WAITID test as protection, but function convertForkfdWaitFlagsToWaitFlags() has not and it uses WEXITED directly without any security.

For this reason, the build process hangs on my platform and probably also on other systems without support for WEXITED into the system headers.