vpath/VPATH support

VPATH/vpath for ckati/ninja mode is missing.

I am thinking of data structures similar to below and parse a statement for vpath directive and VPATH variable. Then evaluate them and populate properly. And these data would be used in dependency build phase.

struct vpath {
string* pattern;
vector paths;
};

struct VpathInfo {
vector vpaths;
vector paths;
};

VpathInfo vpaths_;

I am not sure at which stage in dependency build phase, dependency node needs to be updated. Could anybody provide any suggestions and idea?