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I noticed this myself as well when building a fresh checkout, without build_installer.py. 
This is because the header file and input grammar have the same timestamp, and which forces the rebuild.
That causes problems on OSX when you build with a different deployment target than the one used to build the python command on $PATH. 
I'm not familiar enough with mercurial to know if there is a way to ensure that the header file has a new timestamp when checking in an updated grammer (IIRC you could do this with SVN by first checking in the grammar file and then the generated files).
I'm +O on updating build_framework.py, it should be safe enough as developers should be carefull enough when updating the grammar file.