libopenrave-core: proper linking of libpcrecpp_LIBRARY_DIRS

Gets rid of the CMake warning on link_directories ("Policy CMP0015 is not set: ...")

It still worked fine for me, because pcre is installed system-wide, so libpcrecpp_LIBRARY_DIRS is either empty or "/usr/lib64" which is included anyway. It might be required for users with local installs of pcre to unconventional paths though.