Show build time

Show build time:

$ ./ninja --buildtime
ninja: no work to do.
build time: 00:00:00.000 (0 ms)

$ ./ninja --buildtime
[23/23] LINK ninja
build time: 00:00:00.113 (113 ms)
Maybe http://www.cplusplus.com/reference/cinttypes/ will provide simpler print format?