Absolute third-party paths in wmkdep, possible false positive on sed

Also reported as http://bugs.openfoam.org/view.php?id=2283

In the wmake rules 'transform', the sed expression is missing a beginning anchor and a trailing slash , which makes it slightly susceptible to a false substitutions.

Additionally, any dependencies within ThirdParty retain their absolute path and are not resolved to $WM_THIRD_PARTY_DIR.