Plug waf into MAKE_CMD and CONFIGURE_CMD so the regular defined targets can be
reused
Always define _MAKE_JOBS so that when bsd.port.mk will stop overwritting
_MAKE_JOBS when parallel jobs are disabled we can enforce -j1 (which is needed
to really disable parallelisation with waf
WAF_CMD has been created to allow one to override the location of the waf script
relatively to WRKSRC
CONFIGURE_TARGET is by default defined to "configure"
ALL_TARGET is by default defined to "build"
INSTALL_TARGET is by default defined to "install"
USES=waf is by default stagedir safe
