Add depfile support to build command with multiple outputs (Fixes: #61)

Add depfile support to build command with multiple outputs (Fixes: #61)

parsers.cpp: allow depfile used at build command with multiple outputs.
graph.cpp: allow depfile used at build command with multiple outputs.
parsers_test.cpp: make the test pass.

As before, the depfile itself can only mention one target, which must
be the first of a build command with multiple outpus.

[There is really no need to mention all the output in the depfile,
because all targets should depend on exactly the same files anyway,
because these targets are built by a single build command.]