I had to install extra-cmake-modules (ECM) manually:
brew install haraldf/kf5/kf5-extra-cmake-modules 2>&1 | tee /tmp/install.log
tar zxvf /Library/Caches/Homebrew/kf5-extra-cmake-modules-5.16.0.tar.xz
cmake ../extra-cmake-modules-5.16.0 -DCMAKE_C_FLAGS_RELEASE=-DNDEBUG -DCMAKE_CXX_FLAGS_RELEASE=-DNDEBUG -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/kf5-extra-cmake-modules/5.16.0 -DCMAKE_BUILD_TYPE=Release -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev -DCMAKE_PREFIX_PATH=/usr/local/Cellar/qt5/5.5.1_2/lib/cmake
I got the cmake parameters out of the "install.log" file that brew created. I had to add the CMAKE_PREFIX_PATH myself, after running it the first time and having it fail to find Qt5 properly on its own.
Once I got the proper version of ECM installed, brew counted it as a fulfilled dependency, and I was able to go on to: