See attached picture. The check is incorrect - In the mingw-builds package win32api.h is under i686-w64-mingw32\include .
Anyhow, I suggest to remove the check altogether (which doesn't make any sense).
To validate the installation, one could instead check for a i686-w64-mingw32 subfolder. Even better would be if it would actually run $INSTALLDIR\bin\g++.exe --version: