Check if -static-{libgcc,libstdc++} flags are supported before passing them

[arrowd]

This fixes build on FreeBSD (with Clang).

[TheAssassin]

I'd rather have a non-static mode which you can opt into. This might lead to situations where static linking is needed but implicitly disabled. The default use case is always static linking with this tool.

[arrowd]

This might lead to situations where static linking is needed but implicitly disabled.

My understanding is that clang simply does not support -static-{libgcc,libstdc++} flags, but only -static, which is enough to build a statically-linked executable. I just verified that the executable I built is statically linked indeed.

[TheAssassin]

Oh, now I see why you are doing it like that. In fact, there's a comment missing to explain exactly that. This kind of knowledge is otherwise going to be forgotten and a regression might be reintroduced. Please add one. See also https://standards.mousepawmedia.com/en/latest/csi.html.

Also, not sure if we should just maybe introduce a clang build job.

[TheAssassin]

I see you requested a review, but your approach has not been described in the file yet. Please fix.

[arrowd]

I reworded the commit message, but not CMakeLists.txt. Now added a comment to CMakeLists.txt too.

[TheAssassin]

Thanks. IMO, such information belongs into the code, not the commit history. Trying to find the motivation of changes in the commit history is merely a waste of time. Let's merge this!