[sigc] Using GitHub Actions

[Murray Cumming <murrayc murrayc com>]

I have signed the libsigcplusplus project up to the GitHub Actions
beta, so we can have automatic builds whenever commits are pushed
(continuous integration).
https://github.com/libsigcplusplus/libsigcplusplus/actions

I've set it up to build both the cmake and the autotools builds, and to
try g++ 7, 8, and 9, and clang++ 5, 6, 7, and 8.
https://github.com/libsigcplusplus/libsigcplusplus/tree/master/.github/workflows

Now, whenever someone submits a PR, or merges/pushes to master, these
build checks will run.

(We'll see if GitHub complain that we are spending too many of their
resources.)

Maybe someone would like to create a PR to build with MSVC on Windows
too? See, for instance:
https://help.github.com/en/articles/virtual-environments-for-github-actions#supported-virtual-environments-and-hardware-resources


-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com