Default to /usr/local prefix

It is annoying to default to /usr/bin, so do what is more customary
and default to /usr/local/bin.
Be more strict with warnings

Enable -Werror, to make the build fail if there are any warnings.
Since we are using deprecated Xlib calls, we have to disable that
warning though. The more important warnings won't get lost in the
noise anymore.
Fix a printf warning
Eliminate empty print message warning

GCC warned about the print string being empty, so let's make it
non-empty.