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.

While here:
 - Add c6 libglu to Mk/bsd.linux-apps.mk
 - Add to emulators/linux-c6 as dependency, bump PORTREVISION

Differential Revision:	https://reviews.freebsd.org/D841
Approved by: swills (mentor)
Approved by: portmgr (bapt)


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@370278 35697150-7ecd-e111-bb59-0022644237b5