New interface for passing build information among specs (#1875)

- Added a new interface for Specs to pass build information
  - Calls forwarded from Spec to Package are now explicit
  - Added descriptor within Spec to manage forwarding
  - Added state in Spec to maintain query information
  - Modified a few packages (the one involved in spack install pexsi) to showcase changes

- This uses an object wrapper to `spec` to implement the `libs` sub-calls.
  - wrapper is returned from `__getitem__` only if spec is concrete
  - allows packagers to access build information easily

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

@@ -116,6 +116,18 @@ def url_for_version(self, version):
         return "http://www.open-mpi.org/software/ompi/v%s/downloads/openmpi-%s.tar.bz2" % (
             version.up_to(2), version)
 
+    @property
+    def libs(self):
+        query_parameters = self.spec.last_query.extra_parameters
+        libraries = ['libmpi']
+
+        if 'cxx' in query_parameters:
+            libraries = ['libmpi_cxx'] + libraries
+
+        return find_libraries(
+            libraries, root=self.prefix, shared=True, recurse=True
+        )
+
     def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
         spack_env.set('MPICC',  join_path(self.prefix.bin, 'mpicc'))
         spack_env.set('MPICXX', join_path(self.prefix.bin, 'mpic++'))