Could not find a configuration file for package "ECM" that is compatible with requested version 1.5.0
Try to clone, compile and install the extra-cmake-modules KDE project.
git clone https://invent.kde.org/frameworks/extra-cmake-modules.git
cd extra-cmake-modules
mkdir build && cd build
cmake ..
make && sudo make install
In my case I simply had to install the extra-cmake-modules package.
sudo pacman -S extra-cmake-modules