OpenGL package: always provides gl (#12158)
OpenGL package: always provides gl (#12158)

With this PR, a user can designate older versions of OpenGL as an
external Spack package, and dependents can use that installation
as long as they do not have version requirements.

MacOS currently comes with OpenGL 2.1; there is currently no
'provides' directive in the OpenGL Spack package which specifies a
gl version provided for versions earlier than OpenGL 3.3, so earlier
versions of OpenGL are not considered to provide any version of gl.
This PR records that any version of OpenGL provides 'gl' (which is
sufficient for any package that does not require a specific version
of gl).
LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack