hdf5: patch out mpicxx lib when +mpi~cxx fixes #12586 (#12640)

var/spack/repos/builtin/packages/hdf5/h5public-skip-mpicxx.patch

@@ -0,0 +1,11 @@
+--- a/src/H5public.h	2019-08-28 18:51:39.393781356 -0400
++++ b/src/H5public.h	2019-08-28 20:59:50.315181711 -0400
+@@ -57,6 +57,8 @@
+ #   include <stddef.h>
+ #endif
+ #ifdef H5_HAVE_PARALLEL
++#   define MPICH_SKIP_MPICXX 1
++#   define OMPI_SKIP_MPICXX 1
+ #   include <mpi.h>
+ #ifndef MPI_FILE_NULL		/*MPIO may be defined in mpi.h already       */
+ #   include <mpio.h>

var/spack/repos/builtin/packages/hdf5/package.py

@@ -111,6 +111,11 @@ class Hdf5(AutotoolsPackage):
           sha256='57cee5ff1992b4098eda079815c36fc2da9b10e00a9056df054f2384c4fc7523',
           when='@1.10.2%gcc@8:')
 
+    # Disable MPI C++ interface when C++ is disabled, otherwise downstream
+    # libraries fail to link; see https://github.com/spack/spack/issues/12586
+    patch('h5public-skip-mpicxx.patch', when='+mpi~cxx',
+          sha256='b61e2f058964ad85be6ee5ecea10080bf79e73f83ff88d1fa4b602d00209da9c')
+
     filter_compiler_wrappers('h5cc', 'h5c++', 'h5fc', relative_root='bin')
 
     def url_for_version(self, version):