fix for GREP_OPTIONS

When using GREP_OPTIONS to supply default options to grep, the buildsystem might get broken (For example adding --color=always breaks it)
This patch will empty the GREP_OPTIONS to prevent the described (and any other) problems related to GREP_OPTIONS

Signed-off-by: Maarten Bezemer <m.m.bezemer@utwente.nl>

SVN-Revision: 22443