Issue Description:

Trying to build a ninja manifest that declares an output file to a nonexistent subdirectory, the ninja implementation of llbuild fails to create the subdirectories, which results in build failure.

This does not match the upstream ninja behavior.

A minimal ninja file to reproduce this:

rule CAT
     command = cat ${in} > ${out}build

subdir/output: CAT input

This build file requires a file named `input` to exist in the directory where this is run.