Added an empty LINKFLAGS option to SConstruct.

Without it, SCons won't respect if LINKFLAGS is specified in a config file.

I created this simple pull request from Hradec's as follows :

> git pull upstream master # get my working copy up to date
> git checkout -b linkFlags # make a branch for this feature
> git fetch https://github.com/hradec/gaffer.git # get hradec's repository, but don't merge
> git cherry-pick fc239cf46cf0b2632a4cd734c5dcc5b56b31bcea # merge one commit
> git push origin linkFlags