Adjust formatting of configure help messages and config.h comments

Adjust formatting of configure help messages and config.h comments

* configure.ac (getcontext, execinfo.h, setjmp.h, GC_BUILTIN_ATOMIC):
Add trailing dot in AC_DEFINE comment message.
* configure.ac (gcj-support, java-finalization, atomic-uncollectable,
disclaim): Remove trailing dot in AC_HELP_STRING help message.
* configure.ac (gcj-support, sigrt-signals, java-finalization,
atomic-uncollectable, redirect-malloc, disclaim, large-config,
handle-fork, munmap, checksums, werror, single-obj-compilation,
gcov, docs): Start AC_HELP_STRING help message with a lowercase letter.
* configure.ac (werror): Wrap help message into AS_HELP_STRING.