find_library(rt) (#620)

CMakeLists.txt

@@ -169,7 +169,8 @@ if(ANDROID)
     set(LIBS ${LIBS} log)
 elseif(UNIX AND NOT MINGW)
     set(LIBS ${LIBS} pthread m dl)
-    if(CMAKE_COMPILER_IS_GNUCC)
+    find_library(RT_LIBRARY rt)
+    if(RT_LIBRARY)
         set(LIBS ${LIBS} rt)
     endif()
 endif()