klee: let user override path to runtime library

When looking for runtime library, look first
into KLEE_RUNTIME_LIBRARY_PATH environment variable.
This allows to use klee not only in 'hardcoded' environment.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Merge pull request #178 from mchalupa/master

klee: let user override path to runtime library