rdmd: Improve Make dependencies format

Now lines are only terminated by '\' if another line comes next. Also the
code that generates the dependencies file was made more readable.
rdmd: Fix error generating Make dependencies file

The dependencies file had an extra empty line after the root file
dependency, which ended up in a Make fatal error.
rdmd: Add --extra-file option

This option includes an extra source or object in the compilation. It is
particularly useful if you need to add an extra object (compiled by
another language maybe) or an extra source (maybe because a .di was
provided) when compiling the program (multiple --extra-file are
allowed).
Merge pull request #154 from leandro-lucarella-sociomantic/extra-file

rdmd: Add --extra-file option