calling `sed -i` is evil, let's use cmake's configure_file() instead

configure_file() can generate a file based on a template, so let's use
this instead of sed inplace. also clean the generated file in the clean
target, so the do not pollute the checkout.
configure_file() for the debian scripts
ignore autogenerated files, we only have to track their templates
create scripts in BINARY_DIR, not SOURCE_DIR
build dkms.conf in the binary dir, less stuff to clean up by hand
generate Makefile.dkms and copy it back to the source dir
Merge pull request #107 from evgeni/configure_file

calling `sed -i` is evil, let's use cmake's configure_file() instead