Ninja: Object file compilation unconditionally depends on artifacts of dependencies

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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When the Ninja generator generates targets for .cc files, it adds dependencies on any libraries that their target depend on. This means that in a parallel build, ninja won't start building any .cc files from one library until all depended-upon libraries have been built -- an unnecessary restriction. The only requirement should be that a target cannot be linked until its required libraries are built.

This severely limits build parallelism in large projects. I manually post-processed a build.ninja file from a medium-size C++ project to remove these dependencies, and a parallel build using icecc on a small cluster was reduced from 1m30s to 1m0s. Looking at a Gantt chart of the build, I could see that all of the cluster cores were kept occupied much better after the fix, especially at the start of the build.