package/libglu: needs host-pkgconf

Fix build error

>>> libglu 9.0.0 Autoreconfiguring
configure.ac:50: error: Could not locate the pkg-config autoconf macros.
  These are usually located in /usr/share/aclocal/pkg.m4. If your macros
  are in a different location, try setting the environment variable
  ACLOCAL="aclocal -I/other/macro/dir" before running autoreconf.
configure.ac:50: the top level
autom4te: /home/buildroot/br4/output/host/bin/m4 failed with exit status: 1

using this defconfig

BR2_TOOLCHAIN_BUILDROOT_GLIBC=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XSERVER_XORG_SERVER=y
BR2_PACKAGE_NVIDIA_DRIVER=y
BR2_PACKAGE_LIBGLU=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>