filter_compiler_path: added the possibility to narrow search path

Following a comment from Todd, the search path for the files listed in
`filter_compiler_wrappers` can now be narrowed. Anyhow, the function
implementation still makes use of `find`, the rationale being that we
have already seen packages that install artifacts in e.g. architecture
dependent folders. The possibility to have a relative search path might
be a good compromise between the previous approach and the one suggested
in the review.

Also: 'ignore_absent' and 'backup' keyword arguments can be optionally
forwarded to `filter_file`.

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

@@ -228,7 +228,8 @@ class Openmpi(AutotoolsPackage):
         'mpif90-vt-wrapper-data.txt',
         'mpif90-wrapper-data.txt',
         'mpif77-vt-wrapper-data.txt',
-        'mpif77-wrapper-data.txt'
+        'mpif77-wrapper-data.txt',
+        relative_root=os.path.join('share', 'openmpi')
     )
 
     def url_for_version(self, version):