graphics/linux-c6-libGLU: add port

[xmj]

Summary

• Add port, requested by scf@, so that users of NVIDIA users can enjoy linux

games and have nice things!

• Add linux-c6-libGLU availability to Mk/bsd.linux-apps.mk, to (among others) unbreak games/linux-nwnclient with CentOS

Commitmsg:

graphics/linux-c6-libGLU: Port of CentOS 6.5's Mesa libGLU

Add port of CentOS 6.5's libGLU library. This allows several games to be used
with the new CentOS ports.

Approved by: swills (mentor)

Test Plan

Builds in poudriere.
http://xmj.me/freebsd/buildlogs/linux-c6-libGLU-9.2.log

Will have to be tested by scf@ if it actually works with linux-nwnclient,
as I neither have that game nor an NVIDIA board.

[swills]

[xmj]

Committed revision 370278.