Fix undefined reference to symbol on tests linking
Fix undefined reference to symbol on tests linking

More precisely: undefined reference to symbol 'pthread_key_delete@@GLIBC_2.2.5

The solution is described [here](http://stackoverflow.com/questions/25617839/undefined-reference-to-symbol-pthread-key-deleteglibc-2-2-5)

Probably someone with more knowledge on CMake can do more elegant
MRPT/mrpt,MRPT/mrpt,MRPT/mrpt,MRPT/mrpt,MRPT/mrpt