findExe: do not look into the "exe" resource explicitely if KDE is installed into /usr. In that case
we know it will be part of the $PATH, and we better honour the ordering of $PATH.

I still want to keep the "exe"-lookup when KDE is in a custom prefix, so that it works [better]
even if $PATH doesn't have the custom prefix.
Kudos to Matthias Fuchs and Daniel Hahler for the complete investigation.

FIXED-IN: 4.6