Commit Briefs
fmt: adjust GCC version check
atomics were added in GCC 4.9: https://gcc.gnu.org/gcc-4.9/changes.html
fmt: disable use of stdatomic on AIX XL C and old GCC
C11 is apparently too new for these systems. Fixes #55.
lib9: make formatting lock-free again
First use of <stdatomic.h>. We will see if any supported systems don't have it yet. (C11 was so last decade.) Fixes #338.