Here's a series with suggestions for openmpi.  I hope the log messages
are sufficiently explanatory, otherwise I can comment.  The hwloc and
valgrind changes are in line with what I'm used to using with RHEL and
Debian packaging.  (I know you won't particularly want to follow them,
but they make sense from the point of view of a user.)

I think the last one will need to be used for gfortran-specific
variants, as suggested on -devel.  That will take the closure back up
somewhat, but what I get now is:

  store item                                                       total    self
  /gnu/store/la6mj9kh7fwws233955wyp80x39ag88w-openmpi-1.10.7         134.1     9.7   7.2%
  /gnu/store/b8ni7680lh6j8z26dam7ki9z6f9y6pnz-hwloc-1.11.7-nogui      89.9     2.9   2.1%
  /gnu/store/h7mx27bl0wynlz8vjszzykqqldccfwm5-ncurses-6.0             74.3     5.7   4.2%
  /gnu/store/w1mrskd2ddgvkr727r9241g8dlkf0rlf-gfortran-5.4.0-lib      73.0    34.5  25.7%
  /gnu/store/lsidb1rk8z24c516pqw99anm57cpm8r1-numactl-2.0.11          68.9     0.3   0.2%
  /gnu/store/4vdik5cc02yh2hypwnwi6n6799j6srgn-libpciaccess-0.13.5     68.7     0.1   0.1%
  /gnu/store/dhc2iy059hi91fk55dcv79z09kp6500y-gcc-5.4.0-lib           68.6    30.1  22.4%
  /gnu/store/k7029k5va68lkapbzcycdzj7m5bjb4b8-bash-4.4.12             50.9     5.4   4.1%
  /gnu/store/hvyk1qyph1hihfmym1w271ygp84adb0v-readline-7.0            45.5     1.3   1.0%
  /gnu/store/q1x4v3x8v2g59d244hl7k0i1n4h83c9a-ncurses-6.0             44.2     5.7   4.2%
  /gnu/store/rmjlycdgiq8pfy5hfi42qhw3k7p6kdav-glibc-2.25              38.5    37.1  27.7%
  /gnu/store/02426nwiy32cscm4h83729vn5ws1gs2i-bash-static-4.4.12       1.4     1.4   1.1%
  total: 134.1 MiB