Fix for llvm 4.0.0 on centos (#3904)

* Fix for llvm 4.0.0 on centos

This addresses https://github.com/LLNL/spack/issues/3791

* Only enable this option if on linux

* Change condition to satisfy standard

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

@@ -389,6 +389,9 @@ def install(self, spec, prefix):
                 raise SpackException(
                     'The lldb variant requires the `+clang` variant')
 
+        if spec.satisfies('@4.0.0:') and spec.satisfies('platform=linux'):
+            cmake_args.append('-DCMAKE_BUILD_WITH_INSTALL_RPATH=1')
+
         with working_dir('spack-build', create=True):
             cmake(*cmake_args)
             make()