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/mvapich2/package.py

@@ -107,7 +107,9 @@ class Mvapich2(AutotoolsPackage):
     depends_on('libpciaccess', when=(sys.platform != 'darwin'))
     depends_on('cuda', when='+cuda')
 
-    filter_compiler_wrappers('mpicc', 'mpicxx', 'mpif77', 'mpif90', 'mpifort')
+    filter_compiler_wrappers(
+        'mpicc', 'mpicxx', 'mpif77', 'mpif90', 'mpifort', relative_root='bin'
+    )
 
     def url_for_version(self, version):
         base_url = "http://mvapich.cse.ohio-state.edu/download"