wmtime: Remove unused variables.

This patch removes the unused variables nexttime and old_time_struct to avoid a
warning when -Wunused-but-set-variable is passed.
wmtime: Use rm -f in make clean.

This patch uses the -f option for rm in make clean so that no error occurs when
make clean is run while no object or binary files exist.  (It also adds a
newline at the end of the Makefile thanks to emacs.)
wmtime: Update FSF address.
wmtime: Update GPLv2 from http://www.gnu.org/licenses/old-licenses/gpl-2.0.txt.
wmtime: Update README contact information; remove trailing whitespaces.