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"

etc/spack/defaults/packages.yaml

@@ -23,7 +23,8 @@ packages:
       daal: [intel-daal]
       elf: [elfutils]
       fftw-api: [fftw]
-      gl: [mesa, opengl]
+      gl: [mesa+opengl, opengl]
+      glx: [mesa+glx, opengl]
       glu: [mesa-glu, openglu]
       golang: [gcc]
       ipp: [intel-ipp]