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/mrtrix3/package.py

@@ -19,7 +19,7 @@ class Mrtrix3(Package):
 
     depends_on('python@2.7:', type=('build', 'run'))
     depends_on('py-numpy', type=('build', 'run'))
-    depends_on('mesa-glu')
+    depends_on('glu')
     depends_on('qt+opengl@4.7:')
     depends_on('eigen')
     depends_on('zlib')