Cannot build cairo-clock: undefined reference to symbol 'sinf@@GLIBC_2.2.5'

Hello!

I try to build _cairo-clock_ under Linux and I get this error:

```
/usr/bin/ld: cairo_clock-cairo-clock.o: undefined reference to symbol 'sinf@@GLIBC_2.2.5'
/usr/bin/ld: /usr/lib64/libm.so.6: error adding symbols: DSO missing from command line
```

I believe I should add **-lm** somewhere, but as I don't have much experience with _autotools,_ I don't know how to do.