sys/cdefs.h is not meant to be used by applications, and is not present in musl sys/queue.h is a self-contained header-only library, and as such is not present in musl These are used in tmux-2.1 (and at least one in tmux-2.0). Bug reported upstream and has been patched in git in commit 0a2ef2b (see URL) Possibly backport said commit?
Created attachment 417782 [details, diff] Backported patch to tmux-2.1 This patch is the restriction of the upstream commit to compat/vis.h. (The compilation of version 2.0 fails for different (related) reasons. Since 2.0-r4 is unstable while 2.1 is stable I wouldn't bother adding a patch for version 2.0.)
By now only versions 2.1 and 2.2 are in the tree. The bug is fixed upstream in version 2.2. Fox version 2.1 the patch from comment 1 is still necessary.
