Several nix fixes

[vssukharev]

Recent development shell didn't really provide the working one so that it wasn't possible to build c3c manually. The commit is fixing this.

[tomaskallup]

I can build it just fine using the shell, what I'm not sure about is the comment about usage for C3_CMAKE_FLAGS, the way I build the binary is following:

$ mkdir build
$ cd build
$ cmake .. $C3_CMAKE_FLAGS
$ cmake --build .

[vssukharev]

I have no idea how to do it in other way. We should declare additional information about LLD, cause cmake can't find it. Hopefully, we don't need patches: just additional environment variable.

[lerno]

Thank you!