mesa: re-written to new meson build system (#10482)

* mesa: re-writen to new meson build system

* xorg-server: Remove unneeded mesa dependency

* glx: provide the virtual GLX package

* Convert most "mesa" dependencies to use virtual gl and glx dependencies

* mesa: Switch to always building from the git repo instead of a tarball

* Use gl and glx version ranges instead of exact versions

* glx: provide a default package

* add auto to swr variant

* Revert "add auto to swr variant"

This reverts commit 2e7cd8a8ac.

* update mesa package to version 19.0

* set +glx by default only when running on linux

* fix style issues in geant4 package

* remove virtual package "egl"

var/spack/repos/builtin/packages/opengl/package.py

@@ -3,6 +3,8 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
+import sys
+
 from spack import *
 
 
@@ -18,6 +20,9 @@ class Opengl(Package):
     provides('gl@:4.1', when='@4.1:')
     provides('gl@:3.3', when='@3.3:')
 
+    if sys.platform != 'darwin':
+        provides('glx@1.4')
+
     # Override the fetcher method to throw a useful error message;
     # fixes GitHub issue (#7061) in which this package threw a
     # generic, uninformative error during the `fetch` step,