Create custom PATH that texlive requires in its module. (#8064)

* Create custom PATH that texlive requires in its module.

* Updating texlive to 2018 distribution.

* Using platform instead of sys for easier portability across linux and osx.

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

@@ -24,6 +24,7 @@
 ##############################################################################
 from spack import *
 import os
+import platform
 
 
 class Texlive(Package):
@@ -44,8 +45,8 @@ class Texlive(Package):
     # itself is stable.  Don't let that fool you though, it's still
     # installing TeX **LIVE** from e.g. ctan.math.... below, which is
     # not reproducible.
-    version('live', '8f8fc301514c08a89a2e97197369c648',
-            url='ftp://tug.org/historic/systems/texlive/2017/install-tl-unx.tar.gz')
+    version('live', '946701aa28ca1f93e55e8310ce63fbf8',
+            url='ftp://tug.org/historic/systems/texlive/2018/install-tl-unx.tar.gz')
 
     # There does not seem to be a complete list of schemes.
     # Examples include:
@@ -65,6 +66,10 @@ class Texlive(Package):
 
     depends_on('perl', type='build')
 
+    def setup_environment(self, spack_env, run_env):
+        suffix = "%s-%s" % (platform.machine(), platform.system().lower())
+        run_env.prepend_path('PATH', join_path(self.prefix.bin, suffix))
+
     def install(self, spec, prefix):
         # Using texlive's mirror system leads to mysterious problems,
         # in lieu of being able to specify a repository as a variant, hardwire