coroutines are required for some of the example about libunifex

[lewissbaker]

With #12 it should now auto-detect whether coroutine support is present and avoid including/defining the parts that depend on coroutines.

You can override this auto-detection rules by adding -DUNIFEX_HAVE_COROUTINES=0 to the CMAKE_CXX_FLAGS variable on the command-line.

[lewissbaker]

This has now been merged.
Define -DUNIFEX_NO_COROUTINES=1 to disable coroutines (it should auto-detect by default but you can override by defining this macro).

[ericniebler]

@dietmarkuehl I've beefed up the cmake scripts to do a better job of auto-detecting what features are available, including both coroutines and <memory_resource>. Give it a spin.

[dietmarkuehl]

The code certainly builds with gcc. I haven't managed to build a recent LLVM but I'd believe it is working there, too. There were also some other changes you made such that everything is building with gcc!

Thank you very much!