zoltan: rely on MPI wrappers instead of guessing MPI libraries (#8986)

* zoltan: only add gussed MPI libs if no MPI wrappers are used

- if MPI-wrappers are used for compilation, we can assume that
  linking works without manually specifying MPI libs
  (guessing may result in wrong libs, cf. #8979)
- thus, only guess the NPI libs and add them explicitly if no
  MPI-wrappers are used
- use llnl.util.filesystem.find_libraries instead of a locally
  defined routine to guess the MPI libs if needed
  (cf. #8979)

* zoltan: rely on MPI-wrappers to know the required MPI libs

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

@@ -87,16 +87,11 @@ def install(self, spec, prefix):
 
             config_args.append('--with-mpi={0}'.format(spec['mpi'].prefix))
 
-            mpi_libs = self.get_mpi_libs()
-
-            # NOTE: Some external mpi installations may have empty lib
-            # directory (e.g. bg-q). In this case we need to explicitly
-            # pass empty library name.
-            if mpi_libs:
-                mpi_libs = ' -l'.join(mpi_libs)
-                config_args.append('--with-mpi-libs=-l{0}'.format(mpi_libs))
-            else:
-                config_args.append('--with-mpi-libs= ')
+            # NOTE: Zoltan assumes that it's linking against an MPI library
+            # that can be found with '-lmpi' which isn't the case for many
+            # MPI packages. We rely on the MPI-wrappers to automatically add
+            # what is required for linking and thus pass an empty list of libs
+            config_args.append('--with-mpi-libs= ')
 
         # NOTE: Early versions of Zoltan come packaged with a few embedded
         # library packages (e.g. ParMETIS, Scotch), which messes with Spack's
@@ -135,18 +130,3 @@ def install(self, spec, prefix):
     def get_config_flag(self, flag_name, flag_variant):
         flag_pre = 'en' if '+{0}'.format(flag_variant) in self.spec else 'dis'
         return '--{0}able-{1}'.format(flag_pre, flag_name)
-
-    # NOTE: Zoltan assumes that it's linking against an MPI library that can
-    # be found with '-lmpi,' which isn't the case for many MPI packages.  This
-    # function finds the names of the actual libraries for Zoltan's MPI dep.
-    def get_mpi_libs(self):
-        mpi_libs = set()
-
-        for lib_path in glob.glob(join_path(self.spec['mpi'].prefix.lib, '*')):
-            mpi_lib_match = re.match(
-                r'^(lib)((\w*)mpi(\w*))\.((a)|({0}))$'.format(dso_suffix),
-                os.path.basename(lib_path))
-            if mpi_lib_match:
-                mpi_libs.add(mpi_lib_match.group(2))
-
-        return list(mpi_libs)