CMake install directory wrongly specified

[matthew-brett]

I am getting an error at install time like this:

-- 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".

I tracked this to these lines in cmake_install.cmake (generated file):

IF(NOT CMAKE_INSTALL_COMPONENT OR "${CMAKE_INSTALL_COMPONENT}" STREQUAL "Unspecified")
  FILE(INSTALL DESTINATION "${CMAKE_INSTALL_PREFIX}/." TYPE DIRECTORY FILES "/Users/mb312/dev_trees/minc-toolkit/build/external//Users/mb312/usr/local/lib")
ENDIF(NOT CMAKE_INSTALL_COMPONENT OR "${CMAKE_INSTALL_COMPONENT}" STREQUAL "Unspecified")

I think this comes from the adding SUPERBUILD_STAGING_PREFIX to the output directory around 395. I realize I'm not sure what this is for; I added a commit to remove the use of this prefix to my pull request #6 but maybe this is not the right thing to do.