Gentoo overlay providing libglvnd-free OpenGL support.

This overlay is intended to replace existing packages in the Gentoo Portage tree with versions that do not require the "libglvnd" library (package media-libs/libglvnd), so that users who only use a single OpenGL implementation do not need to install libglvnd. This overlay provides a dummy libglvnd-10000 package which installs nothing and adds a dependency on virtual/opengl, to satisfy packages which depend on libglvnd as a proxy for an OpenGL implementation.

Because this overlay necessarily lags behind the Portage tree, updates to overlaid packages which have not been synced into this overlay may cause cause emerge --update to unintentionally switch to the libglvnd-enabled version. At present, media-libs/mesa will fail at the configuration stage if libglvnd is not actually installed and x11-base/xorg-server will raise a dependency error if x11-drivers/nvidia-drivers[libglvnd] is installed, but x11-drivers/nvidia-drivers will build successfully and may disrupt OpenGL use as a result. Users may wish to explicitly mask the upstream versions of these packages to avoid running into these problems, by adding the following lines to /etc/portage/package.mask (or a file in that directory, if it is a directory):

media-libs/mesa::gentoo
x11-base/xorg-server::gentoo
x11-drivers/nvidia-drivers::gentoo

libglvnd support is still available in packages as an option. To use it without removing this overlay, set USE=libglvnd in make.conf and add a package mask for media-libs/libglvnd::noglvnd.

This overlay can be installed with the following command:

eselect repository add noglvnd git https://github.com/achurch/noglvnd.git

Alternatively, copy noglvnd.conf into /etc/portage/repos.conf (and change the location field as desired).

For background, see: https://bugs.gentoo.org/show_bug.cgi?id=728286