mumps: install mpiseq headers for '~mpi'

Prior to this commit, spack installs a library called `libmpiseq` into
`spec['mumps'].prefix.lib` when it builds MUMPS without MPI. However, it
does not also install the headers corresponding to this library, so it
is impossible to compile source files that depend on function calls
implemented in this library. This commit fixes this problem by
installing these headers, which are needed by packages (e.g., IPOPT)
that depend on these headers.

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

 from spack import *
-import os, sys
+import os, sys, glob
 
 class Mumps(Package):
     """MUMPS: a MUltifrontal Massively Parallel sparse direct Solver"""
@@ -164,10 +164,13 @@ def install(self, spec, prefix):
 
         install_tree('lib', prefix.lib)
         install_tree('include', prefix.include)
-        if '~mpi' in spec:
+
+        if '~mpi' in spec:            
             lib_dsuffix = '.dylib' if sys.platform == 'darwin' else '.so'
             lib_suffix = lib_dsuffix if '+shared' in spec else '.a'
             install('libseq/libmpiseq%s' % lib_suffix, prefix.lib)
+            for f in glob.glob(join_path('libseq','*.h')):
+                install(f, prefix.include)
 
         # FIXME: extend the tests to mpirun -np 2 (or alike) when build with MPI
         # FIXME: use something like numdiff to compare blessed output with the current