Update TODO based on completed tasks.

Update TODO based on completed tasks.

* TODO (AC_PROG_INSTALL takes multiple files): Done.
(AC_GNU_SOURCE deprecation): Done, see AC_USE_SYSTEM_EXTENSIONS.
(AC_COMPILE_IFELSE documentation): Done.
(Tracing builtins): Done, now that we require M4 1.4.5.
(AC_PROG_CC_POSIX suggestion, providing header files)
(AC_TYPE_SIGNAL): Not needed; gnulib's approach is better.
(cache consistency): Done with precious variables.

Signed-off-by: Eric Blake <ebb9@byu.net>