openmpi: add package-specific 'headers' property (#7267)

* [OpenMPI] Add the 'headers' property. This removes some redundant
headers from sub-directories, returned by the default '.headers'
handler.

* [OpenMPI] In the .headers property, add a fallback to search all of
prefix for the mpi.h header. Recommended by @junghans to support some
external configurations, see #7268.

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

@@ -220,6 +220,13 @@ def url_for_version(self, version):
         url = "http://www.open-mpi.org/software/ompi/v{0}/downloads/openmpi-{1}.tar.bz2"
         return url.format(version.up_to(2), version)
 
+    @property
+    def headers(self):
+        hdrs = HeaderList(find(self.prefix.include, 'mpi.h', recursive=False))
+        if not hdrs:
+            hdrs = HeaderList(find(self.prefix, 'mpi.h', recursive=True))
+        return hdrs or None
+
     @property
     def libs(self):
         query_parameters = self.spec.last_query.extra_parameters