Fix broken build when PATH is defined explicitely

Build was broken when called like this:

PATH="/usr/sbin:/usr/bin:/sbin:/bin" dpkg-buildpackage -us -uc  -sa -rfakeroot

This also broke builds with Jenkins Debian Glue.
Replace $$PWD by $(CURDIR) in debian/rules

Lintian complained about this.
Merge pull request #4 from mark0n/path

Fix build issue with the PATH variable