Make it possible to use ECM without installation

Frederik Gladhorn requested to merge frederik/no_installation into master

extra-cmake-modules is perfectly relocatable and doesn't require any configuration. This allows projects to use KDE frameworks as simple git submodules when using cmake.

The motivation is to make life easy for Windows and macOS developers who only need one or two frameworks and do not have ECM installed already.

Example finding ecm when it is a git submodule: set(ECM_PATH "${CMAKE_SOURCE_DIR}/thirdparty/extra-cmake-modules") find_package(ECM 5.79.0 REQUIRED PATHS ${ECM_PATH})

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