teach toolbox to use a relative path to generate src/version.h

When toolbox is called from the makefile, src/ is the working directory
and it fails to create src/version.h.
By using a path relative to its own location, it can be called from
anywhere in the project.

Signed-off-by: Patryck Rouleau <pfrouleau@gmail.com>
Merge pull request #33 from pfrouleau/fix_version_h_generation

teach toolbox to use a relative path to generate src/version.h