lib/functions.in: added support for 'waf' build system
Following new variables have been introduced and are usable in conf files:
runwaf set to '0' if you want to skip running 'waf' build system.
wafscript script to run for waf based builds
(default = './waf')
wafconfopt options for 'waf configure'
(default = 'configure --prefix=/$prefix')
wafbuildopt options for 'waf build'
(default = 'build')
wafinstopt options for 'waf install'
(default = 'install --destdir=$root --force')