Log Message:
Add support for WAF_HAS_MANDIR (by default "yes").

Always forcing `--mandir=' in WAF_ARGS can be problematic for waf-s that do
not support that (e.g. nanomsgxx, ardour). WAF_HAS_MANDIR can be set
per-package to "no" for these packages. Pointed out by <kamil>.

ok <wiz>