This package was built from a "_orig.tar.gz" tarball that had all Autotools-generated files removed from the upstream tarball. This was done using the "orig" target that is defined in "Makefile.am", which is part of the original upstream source package. The package is built using autoreconf, so all of those removed files are re-created upon building. Removing those files helps porting to new architectures and ensures building with the latest versions of all Autotools programs. By default, Autotools will convert a lex/flex ".l" file into a ".c" file, and leave the .c file in place during "make clean" or "make distclean". Therefore, debian/clean removes the generated ".c" files in the src directory.