build: clock_gettime() is in libc on BSDs

CMakeLists.txt

@@ -181,7 +181,7 @@ ELSEIF(MINGW)
     add_definitions(-D_WIN32_WINNT=0x0600)
 ELSEIF(WIN32)
     set(PLATFORM_LIBRARIES winmm ws2_32)
-ELSE()
+ELSEIF(CMAKE_SYSTEM_NAME MATCHES "^(Linux|kFreeBSD|GNU|SunOS)$")
     set(PLATFORM_LIBRARIES rt)
 ENDIF (APPLE)