add support for sticking generated files in the build directory

add support for sticking generated files in the build directory
instead of with the source code.  This will hopefully be especially
useful when we're generating dozens of files when we flesh out the
object description stuff.
remove generated files from the source tree.  python is required
to build now.

base/trace.hh:
    no need for the underscore in the name
base/traceflags.py:
    clean up code

--HG--
extra : convert_revision : f68af8c3460eb7e73a1defaea3081a02ad7db33c