Pullup a fix from -current pkgsrc to the netbsd-1-6 pkgsrc branch

Pullup a fix from -current pkgsrc to the netbsd-1-6 pkgsrc branch
for pkgsrc/graphics/glu/Makefile.

Requested by Manuel Bouyer.

"I discovered a problem with graphics/glu: its buildlink.mk checks if
glu is already part of base system, but not the Makefile itself, so
during a bulk build the base system libglu will be overwritten by the
package.  Our base system has libglu.1 where the package installs
libglu.3, in the same place.  This results in some packages having an
effective dependancy to the glu package (because of the lib majuor
number difference), which is not recorded in the package.  So
installing e.g.  kde2 won't pullup glu, but it won't run because of
the missing libglu.so.3 The attached patch solves this problem.  The
code comes from pkgsrc-current,"