package/libglew: do not depend on libglu

"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

This patch prevents libglu being added to the "Requires:" section of
glew.pc fixing libglew detection in the supertuxkart package

-- Checking for module 'glew>=2.1'
--   Package 'glu', required by 'glew', not found

after libglu is removed as dependency of supertuxkart.

Supertuxkart already uses -DGLEW_NO_GLU:
https://github.com/supertuxkart/stk-code/blob/master/CMakeLists.txt#L224



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