When using Slint via FetchContent, don't print a message on every re-configure

[tronical]

Require CMake 2.4 to intregate CMake FetchContent and find_package, according to https://cmake.org/cmake/help/latest/guide/using-dependencies/index.html#fetchcontent-and-find-package-integration

This way FetchContent_MakeAvailable will first try to call find_package(Slint) and if that fails then fall back to building via FetchContent.

Fixes #16

[ogoffart]

Maybe you want to use OVERRIDE_FIND_PACKAGE ? This seems to do what is asked in #16

Edit: Never mind, this is incompatible with FIND_PACKAGE_ARGS and i prefer the behavour of FIND_PACKAGE_ARGS

[tronical]

I'll defer this, as what we have does work after all and doesn't require a newer cmake.

[tronical]

Edit: Keeping it open so that we won't forget about it :)