package/supertux: remove libglu dependency

"The inclusion of GLU is historical. [...] It's not needed for GLEW to
 build or run, but it's opt-out that reason. I'd recommend using
 -DGLEW_NO_GLU since GLU is rarely seen in modern codebases nowadays."
https://github.com/nigels-com/glew/issues/192#issuecomment-411641657



Each package using glew.h has to decide whether to use the opt-out for
glu or not. Since supertux does not need libglu we can safely remove
the dependency by adding -DGLEW_NO_GLU to TARGET_CXXFLAGS.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>