libsdl: Removed dependency on libglu

libglu requires libGL. If we do not support virtual/libgl
then we have to remove libglu as well.
x86-pcsim builds fail if dependency on libglu is not removed.

Signed-off-by: Mikhail Durnev <mikhail_durnev@mentor.com>
Merge pull request #138 from mdurnev/mdurnev/libsdl

libsdl: Removed dependency on libglu