CGAL 5.0 Support

Hello,

While upgrading CGAL to 5.0 on homebrew, I found out several issues with graph-tools:

• the configure file is not compatible with a header-only CGAL as it explicitely looks for libraries, which don't exist anymore
• the m4 file for finding boost coroutine is broken, the test is missing an include

I proposed a patch on homebrew, but it will only work for CGAL 5.0. You can still check it to get some inspiration if you want, and can find it here