Build shared libraries with MPICH

I believe this is required. Without this, I cannot build Boost with MPICH and I get the following error:
```
gcc.link.dll bin.v2/libs/mpi/build/gcc-4.8.1/release/threading-multi/libboost_mpi.so.1.53.0
/usr/bin/ld: /home/users/xbesseron/.local/easybuild/software/MPICH2/3.0.4-GCC-4.8.1/lib/libmpich.a(attr_get.o): relocation R_X86_64_32 against `MPIR_ThreadInfo' can not be used when making a shared object; recompile with -fPIC
/home/users/xbesseron/.local/easybuild/software/MPICH2/3.0.4-GCC-4.8.1/lib/libmpich.a: could not read symbols: Bad value
collect2: error: ld returned 1 exit status
```