Pass more paths explicitly to configure
These paths are otherwise looked up from PATH at build-time and embedded into shipped files. This means that for example on merge-usr systems tools are found in /usr/bin that on non-merged systems live in /bin.
When building in a local dirty environment, they could of course be picked up from any other place also in PATH, like /usr/local/bin.
The additions in this commit fixes only the part of the reproducability problem related to merged-usr vs non-merged systems (but the package still doesn't build reproducably in general) and it only caters for the paths affected by merged-usr.... There might still be other paths that should also be specified explicitly that could be affected for other reasons than merged-usr in a local dirty build environment.