Configured CMake to copy and update dists.dss

[adrianlut]

dbgen expects the dists.dss file inside its working directory. To facilitate out-of-source builds, CMake is now instructed to copy the file to the build folder where dbgen is located after compiling.

I also updated README.md and .gitignore to reflect this change.

[eyalroz]

Why would dbgen be run with the working directory being the build directory?

[adrianlut]

I think this change is mostly about defaults. As far as I know, out-of-source builds are generally considered good practice. Since dists.dss is a configuration file required at runtime, it belongs to the "distribution" of the runtime and therefore should be copied next to the compiled binary. In my opinion at least.

As to why dbgen would be run from within the build directory: this is just the default of my IDE and, as far as I know, it is also the default for other IDEs and languages, isn't it? Also, the compiled program depending on a file in the source tree seams weird to me.

Maybe the next change should be to change the default behavior of expecting the dists.dss file in the working directory to expecting it in the executable directory.