GitLab

* configure.ac: With clang, check for and use -Wno-switch,
-Wno-tautological-constant-out-of-range-compare and -Wno-pointer-sign.

* conf_post.h(assume): Use __builtin_unreachable for clang.

* src/filelock.c (lock_file_1): Rearrange to remove compiler warning
about excess arguments to snprintf.