Bug#820046: guitarix: does not respect DEB_BUILD_OPTIONS=parallel max value

[Hermann Meyer]

If you use the DEB_BUILD_OPTIONS=parallel flag, you must past the 
$(NUMJOBS) somehow to the waf configure flags.
https://www.debian.org/doc/debian-policy/ch-source.html#s-debianrules-options
guitarix wscript accept the -j$(NUMJOBS) parameter.

regards
hermann