add a --bootstrap mode for configure.py

Instead of bootstrapping through a separate script, instead make
configure.py able to either generate a build.ninja *or* just execute
all the computed commands to build a ninja binary.

Note: this isn't complete.
- there's one XXX 
- I'm not happy about the sys.exit() bit
- need to replace/update bootstrap.py, docs, etc.


The `n.variable` call below adds `'$ '` to the end of the string, which confuses `_expand()`. This doesn't work right if any of CXX, AR, CFLAGS, LDFLAGS is set in the env.