default built_ext behavior does not automatically clean first
default built_ext behavior does not automatically clean first

added '-c', --clean option to be used with -b to clean then build.
This is a more efficient way to build so give user control over when to
clean before doing a complete build.

rmcgibbo/conda-build,mwcraig/conda-build,ilastik/conda-build,mwcraig/conda-build,dan-blanchard/conda-build,mwcraig/conda-build,ilastik/conda-build,dan-blanchard/conda-build,sandhujasmine/conda-build,ilastik/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,frol/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,frol/conda-build,dan-blanchard/conda-build,frol/conda-build