proactively create build directories

This issue is moved from: https://www.nsnam.org/bugzilla/show_bug.cgi?id=2806

Summary: Some build scripts will fail if the configure target directory (in bake's case, configured to 'build/lib') does not exist. We can work around this on a package-by-package basis by adding a 'mkdir -p build/lib' pre_installation command, but it would be cleaner to just have bake do this at the start of the build process.

Suggest that './bake.py build' will create, using 'mkdir -p', the 'build/lib', 'build/share', and 'build/bin' directories, and remove all such mkdirs from bakeconf.xml.