CI coverage fixes (5)

CI coverage fixes (5)

Add comment about async signal safety for PR boostorg#70

Add comment about async signal safety for PR boostorg#70

Merge pull request boostorg#67 from JanEisenhauer/develop

Collect at most as many frames as requested.

Merge pull request boostorg#67 from JanEisenhauer/develop

Collect at most as many frames as requested.

Add info that the library is proposed for C++20

CI linkage fixes (4)

Updated the docs to better reflect the MinGW situation. Refs boostorg#14

, boostorg#34, boostorg#36

Do not check for _GNU_SOURCE on Windows