Fix bug in MPI error message output

Memory wasn't properly allocated

Also use buffered stderr (clog) for error messages, this
is a workaround specifically for Cray's aprun, which
apparently segfaults when using unbuffered stderr,
leading to incompletely displayed error messages
Add installation directory to search paths for hoomd lib

This fixes a bug where on Blue Waters it was necessary
to explicitly set PYTHONPATH to point to the location of the hoomd
library and launch HOOMD scripts using python instead of hoomd

The reason for this was that aprun copied the hoomd executable
to a different location (outside the installation tree) before
execution, so the automatic deduction of the runtime path failed
Fix cmake syntax warning