This bug is probably specific to Mac OS X installations where Qt is copied into place with "make install."
Qt provides files to support using CMake with Qt-based projects. For definiteness, assume that Qt is ultimately to be installed in DESTDIR. These cmake support files in DESTDIR/lib/cmake require that Qt headers be installed in DESTDIR/include, and CMake will object if it finds the cmake files but not the include files in DESTDIR/include.
It appears that the problem is in a recent change to qt_module.prf caused most headers not to be copied into the include directory upon installation. This change occurred after RC1 and before RC2:
Removing the line
CONFIG -= qt_install_headers #no need to install these as well
seems to fix the problem.