In #26630, I assumed "glu" was needed by glew because it included glu.h, but
actually, glew can be used without glu when GLEW_NO_GLU is defined and this
is documented in the announcement of glew-1.6.0:
> https://www.geeks3d.com/20110430/opengl-glew-1-6-0-available/
> * Define GLEW_NO_GLU for no glu dependency
It is therefore the duty of users of glew to decide if they use glu,
and then they need to have a depends_on("glu").
Thus, move the depends_on("glu") which I changed from "gl" in #26630
to vapor, which itself uses glu as well.
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