Make the port build with the upcoming Qt 4.8.x series.

Since 4.8.0 [1], Qt itself does not automatically include GLU's
headers and libraries with its OpenGL module, so applications which
relied on Qt to get these need to be adjusted.

The upcoming update to x11/qt4-opengl will thus switch to USE_GL=gl,
so we need to add USE_GL=glu for this port.

PORTREVISION bumped since we now explicitly depend on GLU via
USE_GL=glu.

[1] http://qt.gitorious.org/qt/qt/commit/e7eed096a0c33607a7a37baaf06e5952dc9d556b

Approved by: miwi (we need this fixed for the KDE 4.8.3 and Qt 4.8.1 update to land)