Problem:
A per-test Linker search path was added in commit a911fa41 to CMakeLists.txt as
part of fixing Windows builds. Whilst this is silently ignored by ld(1) on
Linux, it doesn't settle well with OS X. Spurious warnings are generated about
missing directories leading to convoluted build logs.

Solution:
Make per-Test LINK_DIRECTORIES() conditional for non-Apple platforms.