librt not available on Mac OS

[gonium]

Linking CXX executable klio-export /usr/local/bin/g++-4.8 -Wall -Wno-deprecated-declarations -Wall -Wno-non-virtual-dtor -g -fomit-frame-pointer -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/klio-export.dir/klio-export.cpp.o -o klio-export ../libklio/libklio.a /usr/local/lib/libboost_test_exec_monitor-mt.a /usr/local/lib/libboost_program_options-mt.a /usr/local/lib/libboost_filesystem-mt.a /usr/local/lib/libboost_system-mt.a /usr/local/lib/libboost_date_time-mt.a /usr/lib/libsqlite3.dylib /usr/local/lib/libboost_test_exec_monitor-mt.a /usr/local/lib/libboost_program_options-mt.a /usr/local/lib/libboost_filesystem-mt.a /usr/local/lib/libboost_system-mt.a /usr/local/lib/libboost_date_time-mt.a /usr/local/lib/libjson.a -lrt /usr/lib/libcrypto.dylib ld: library not found for -lrt

On OSX 10.9, librt is not available. The offending library is statically requested in these modules:

$ grep 'lrt' modules/* FindJson.cmake: list(APPEND JSON_LIBRARY "-lrt") FindRocksDB.cmake: list(APPEND ROCKSDB_LIBRARY "-lrt")

Please replace this by a clean detection method. See #a9fcb22.