avoid warning in gutils.h when using gcc with -Wconversion

Old solution to avoid warning with gcc flag -Wconversion does not
work with gcc >=4.3 since the behaviour for that flag has been
changed, ref. http://gcc.gnu.org/wiki/NewWconversion. Now cast for
__builtin_clzl is required, which itself is declared to return int.

https://bugzilla.gnome.org/show_bug.cgi?id=619026