Set _POSIX_C_SOURCE as we need it for sigemptyset when using muslc
Review Request #2480 — Created June 21, 2023 and submitted
muslc requires _POSIX_C_SOURCE to be set to get sigemptyset and a few other things. I went with 200809L as the value since we're using the C17 standard now and this is the most recent POSIX version that I could find.
alpine-edge-amd64convey plan locally and verified the build was successful.