Add an explicit -lm to the link lines in Makefile.gtk, after two users

Add an explicit -lm to the link lines in Makefile.gtk, after two users
reported on the same day that this is now necessary since up-to-date
GNU tools won't consider it sufficient to have libm be a dependency of
other explicitly referenced libraries if you're directly referring to
the contents of libm yourself.

[originally from svn r9448]