BF: double directory specification for install

Getting error at install time:

    -- Install configuration: ""
    CMake Error at cmake_install.cmake:31 (FILE):
      file INSTALL cannot find
        "/Users/mb312/dev_trees/minc-toolkit/build/external//Users/mb312/usr/local/bin".

where ``/Users/mb312/usr/local`` is my install prefix.  Remove puzzling
extra directory from install output directory spec.