Revert "Revert "Merge pull request #15 from bentian/bug-951565""

Trying to re-land bug 951565 with fix applied.

This reverts commit 55a5f7def3310ba245102a97270daf1c052c058d.
Bug 951565: Set GECKO_PATH if it's not defined

GECKO_PATH might not have been set during the build process. In
this case we default to the directory gecko/.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>