smpicc: don't add -shared when SMPI_PRETEND_CC is on

all mpi programs are compiled in the -shared binary model so that we
can load them with dlopen afterward. But shared binaries cannot be run
by themselves, which is a problem to ./configure that wants to ensure
that mpicc is a real compiler (not a cross-compiler) by executing some
stupid code of its own.

Before the introduction of dlopen as a privatization mechanism, we
used to prevent the binary doing weird things when SMPI_PRETEND_CC was
defined. Now, we extend this mecanism for our compiler wrapers to play
safe in this case.