Clock module: BSD and ubuntu fix

CMakeLists.txt

@@ -93,7 +93,11 @@ check_function_exists(uuidgen HAVE_UUIDGEN)
 # clock_gettime() is not defined on Mac OS X,
 # or own (slow) implementation is in third_party/clock_gettime.c
 #
-check_library_exists(rt clock_gettime "" HAVE_CLOCK_GETTIME)
+if (TARGET_OS_LINUX)
+    check_library_exists(rt clock_gettime "" HAVE_CLOCK_GETTIME)
+else()
+    check_function_exists(clock_gettime HAVE_CLOCK_GETTIME)
+endif()
 
 # Checks for libev
 include(CheckStructHasMember)