Enable highlight executable path as a configuration option

Enable highlight executable path as a configuration option

Allow build-time/run-time configuration of the highlight executable
(must be the one from http://www.andre-simon.de due to assumptions
about parameters and output).  Defaults to previous behavior which
assumes that highlight is available on the server PATH. However, if
this is not the case, the path to the highlight executable can be
configured at build time as a configuration variable

    HIGHLIGHT_BIN = /path/to/highlight

or at runtime by configuring GITWEB_CONFIG

    $highlight_bin = /path/to/highlight

Signed-off-by: Christopher Wilson <cwilson@cdwilson.us>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>