We use math routines inside Cogl, so it's correct to have it in
the LIBADD line. In normal usage something else was pulling in
-lm, but the introspection is relying on linking against the
convenience library.
Based on a patch by: Colin Walters <walters@verbum.org>
Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>