Can't set the path of generated C header (while compiling static or dynamic lib)

[vssukharev]

Steps to reproduce:

1. Create project.json with the following content:

{
  "output" : "build",

  "targets" : {
    "mylib" : {
      "type" : "static-lib",
      "sources" : [ "main.c3" ],
    },
  },
}

2. Create main.c3 with arbitrary content.
3. Run command c3c build mylib.

This command will produce a header mylib.h in root dir. Of course, it would be better to generate everything into build, but also it would be essential to have option like --header-output, which represents the emitted header path.