Build system hacks to build and install the MPI wrappers library when

Build system hacks to build and install the MPI wrappers library when
a suitable mpi.h is found at configure time.  This also adds the
configure flag --with-mpi=/path/to/mpi/install so that libmpiwrap.so
can be built against any given MPI installation.

libmpiwrap.so is built and installed for the primary target only.  As
usual this all involves various unsavoury build-system hacks.
Fortunately they are all in auxprogs/Makefile.am and configure.in
don't interact with any of our existing build-system hacks :-)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@5707