Only add -lrt if librt exists and has symbols.

On Mac OS X, librt does not exist at all. On Linux/amd64, it is
typically empty. Use AC_CHECK_LIB to add librt only when needed.

configure.ac

@@ -65,7 +65,10 @@ if test "$spellcheck" = yes; then
 fi
 
 # Add explicit linking flags
-GUI_LIBS="$GUI_LIBS -pthread -lm -lz -lrt"
+GUI_LIBS="$GUI_LIBS -pthread -lm -lz"
+AC_CHECK_LIB(rt, clock_gettime, [
+    GUI_LIBS="$GUI_LIBS -lrt"
+])
 
 # Checks for header files.
 AC_CHECK_HEADERS([libintl.h locale.h netdb.h stdlib.h string.h sys/socket.h sys/time.h unistd.h fcntl.h])