gnu: Add 'version-prefix' and 'version-major+minor'; use them.

The procedure version-prefix truncates a version string to a certain
number of components. version-major+minor is a special case of this,
which extracts the "major.minor" part of the string. Previously this
was handled by an expression common to several packages.

* guix/utils.scm (version-prefix, version-major+minor): New procedures
* gnu/packages/backup.scm (duplicity): Use version-major+minor.
* gnu/packages/cmake.scm (cmake): Likewise.
* gnu/packages/gnome.scm (libbonobo, libbonoboui): Likewise.
* gnu/packages/gnutls.scm (gnutls): Likewise.
* gnu/packages/mail.scm (gmime): Likewise.
* gnu/packages/mpd.scm (mpd): Likewise.
* gnu/packages/mpi.scm (openmpi): Likewise.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

Merge branch 'master' into core-updates

gnu: openmpi: Add memchecker support.

* gnu/packages/mpi.scm (openmpi) [inputs]: Add valgrind.  Unpropagate
  gfortran.  [arguments]: Add configure flags for memchecker.

gnu: openmpi: Use our hwloc.

* gnu/packages/mpi.scm (hwloc): Propagate numactl.
  (openmpi)[inputs]: Add hwloc.
  [native-inputs]: Add pkg-config.
  [argument]: Add hwloc configure flags.

gnu: Add hwloc.

* gnu/packages/mpi.scm (hwloc): New variable.
  (openmpi): Add TODO comment.

gnu: openmpi: Fix typo in download uri

* gnu/packages/mpi.scm (openmpi)[origin]: Fix typo.

gnu: openmpi: Avoid "open source" verbiage

* gnu/packages/mpi.scm (openmpi)[synopsis,description]: Remove mention
  of "open source".

gnu: Add OpenMPI

* gnu/packages/mpi.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.