When configure detects some NO_XXX or NEEDS_XXX is unneeded, unset
this variable (actually set it to empty string). This allow
autodetection to override the default set in Makefile.
[jc: while at it fixed a thinko in IPv6 detection.]
Signed-off-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <junkio@cox.net>