Introduce a base build.ninja file.

[Thomas Shadwell]

 Hi all – Gerrit was unable to assign a reviewer to my patch, so I'm sending this one to the list :)

https://gn-review.googlesource.com/c/gn/+/15480

I was trying to get gn to build under bazel (so I can build some ninja projects under rules_foreign_cc) and came across this neat solution that uses a phony target to get ninja to immediately rebuild build.ninja, thus allowing the project to be built just by running `ninja`.

Thomas