Run gperf with --output-file instead of output redirection

Run gperf with --output-file instead of output redirection

When missing gperf, the redirection generates an empty file, which must
be manually removed after gperf has been installed. This is difficult
to diagnose, as the produced build error is cryptic.

Use --output-file of gperf instead to avoid creating an empty file if
gperf is missing. This still requires the user to re-run ./configure
after installing gperf, though.