--out does not work with absolute paths

[rbehrends]

If the --out option is provided to nimrod as an absolute path, it will treat it as a relative path regardless. This is because of the following code fragment in extccomp.nim:

      if not noAbsolutePaths():
        exefile = joinPath(splitFile(projectFile).dir, exefile)

The value of noAbsolutePaths() is true if either the --genScript or the --genMapping option is set. However, if either of these options is provided, the compilation process fails, because the C compiler is not given the correct paths to the generated .c files.