Finding libexec files / relocation

[Alex Merry]

Currently, paths to libexec files are hardcoded (or not... see
https://git.reviewboard.kde.org/r/117023/).  This works find unless you
want to relocate anything.

I know that relocating is something that has come up with KDE on Windows
stuff.  Is it something we want to support?  If so, how would we do it?

I think most of the rest of our stuff (including CMake code) supports
relocation, even if you have to set some environment variables to
support it (like LD_LIBRARY_PATH on Linux).

Alex