Fix clean MPI-enabled build

Fixes trac issue [#10632](http://trac.mantidproject.org/mantid/ticket/10632)

A clean build with MPI enabled now completes sucessfully on Ubuntu. On RHEL, however, there seems to be a problem with boost that produces a linker error
```
/tmp/ccQU3mJK.o: In function `main':
start_env.cpp:(.text+0x15): undefined reference to `boost::mpi::environment::environment(bool)'
collect2: ld returned 1 exit status
```
but this is, as far as I can tell, not something we can fix in our code base. I have filed a bug with redhat: https://bugzilla.redhat.com/show_bug.cgi?id=1169501