New version mesa-18.1.2 (#8589)

- compiles with newer LLVM.

  The older mesa-17 versions balk at the changes in the llvm::FastMath
  class, where a member function has been removed.

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

@@ -36,6 +36,7 @@ class Mesa(AutotoolsPackage):
     _oldurlfmt = "https://mesa.freedesktop.org/archive/older-versions/{0}.x/{1}/mesa-{1}.tar.xz"
     list_depth = 2
 
+    version('18.1.2', 'a2d4f031eb6bd6111d44d84004476918')
     version('17.2.3', 'a7dca71afbc7294cb7d505067fd44ef6')
     version('17.2.2', '1a157b5baefb5adf9f4fbb8a6632d74c')
     version('17.1.5', '6cf936fbcaadd98924298a7009e8265d')