Many Macs don't have a right mouse button.
Don't try to use an external RMF.

Most likely the external library won't be compatible with
current IMP, and the configure checks don't look for the
external library's SWIG interface files, so building IMP.rmf
with fail with a "can't find RMF.i" error. Closes #798.
Don't update submodules every time we run cmake.

Updating submodules throws away any changes made to them (e.g. PMI)
making work on a development version of PMI rather difficult.
Closes #799.