ignore the current build directory when packaging
ignore the current build directory when packaging

This makes sure that the current build directory is not packaged
with the regular source tree when `make package_source` is used.

mojoBrendan/fmt,cppformat/cppformat,mojoBrendan/fmt,cppformat/cppformat,dean0x7d/cppformat,mojoBrendan/fmt,cppformat/cppformat,alabuzhev/fmt,alabuzhev/fmt,lightslife/cppformat,dean0x7d/cppformat,alabuzhev/fmt,dean0x7d/cppformat,lightslife/cppformat,lightslife/cppformat