Determining if the include file pthread.h exists failed with the following output:
Change Dir: C:/qtb/bld/qtwebengine/src/CMakeFiles/CMakeTmp

Run Build Command(s):C:/PROGRA~2/MICROS~1/2019/COMMUN~1/Common7/IDE/COMMON~1/MICROS~1/CMake/Ninja/ninja.exe cmTC_f6dbb && [1/2] Building C object CMakeFiles\cmTC_f6dbb.dir\Debug\CheckIncludeFile.c.obj

FAILED: CMakeFiles/cmTC_f6dbb.dir/Debug/CheckIncludeFile.c.obj 

C:\PROGRA~2\MICROS~1\2019\COMMUN~1\VC\Tools\MSVC\1429~1.301\bin\Hostx64\x64\cl.exe  /nologo -DCMAKE_INTDIR=\"Debug\"  /DWIN32 /D_WINDOWS  /Zi /Ob0 /Od /RTC1 -MDd /showIncludes /FoCMakeFiles\cmTC_f6dbb.dir\Debug\CheckIncludeFile.c.obj /FdCMakeFiles\cmTC_f6dbb.dir\Debug\ /FS -c CheckIncludeFile.c

CheckIncludeFile.c(1): fatal error C1083: Cannot open include file: 'pthread.h': No such file or directory
ninja: build stopped: subcommand failed.