README: Adding ExternalProject_Add example

[thopiekar]

Added a short description how I added AppImageKit in one of my projects.

[probonopd]

@TheAssassin as our CMake artist in residence, does this "speak" to you? (For me, the many variables that CMake seems to use everyone makes it hard to understand what it is doing.)

[TheAssassin]

Looks quite a bit cryptic to me, too. I'll have a detailed look next week.

[thopiekar]

Oops, I thought you merged the PR already. I forgot you have permissions to push into my branches if they are used for PRs 😅

[TheAssassin]

So, I suppose this is about building appimagetool and using it in your project, rather than using our (relatively new) libappimage? I'll clarify that a bit then in the README.

[thopiekar]

Ok, feel free to do that. No, I'm not using the libappimage, but I guess it should be easy to prevent CMake to generate Makefiles, which build only libappimage 😉

[luzpaz]

I'm confused on what the PR actually does?

[thopiekar]

Yes, SetProjectDependencies is custom and the BUILD_OS_LINUX variable as well.

[TheAssassin]

This PR has been closed by the original author. No need for reviews or anything.

[TheAssassin]

Unexpected side effect of renaming of master branch. Sorry for the noise. Changing base and reopening.