Fix build error on Ubuntu 13.04.

When application build on Ubuntu 13.04 in usual way:
./autogen
./confugure
make
appears error:
/usr/bin/ld: notebook.o: undefined reference to symbol 'round@@GLIBC_2.2.5' .

Error was reproduced on three different machines.

This pull request try to fix this error (may be not the best way).