BUG: Extension packaging: For SuperBuild extension, ensure fixup happens last

For extension of type "SuperBuild", the external projects to install are
specified by appending build directories to the variable
"CPACK_INSTALL_CMAKE_PROJECTS", then this variable is processed at
packaging time by "CPack".

This means that simply ensuring the fixup script is executed last during
the install of the current project was not sufficient.

Instead, this commit creates a minimal CMake project named
"<ExtensionName>-Fixup" that is including the fixup script. Then, this
project is appending last to the variable "CPACK_INSTALL_CMAKE_PROJECTS".

See SBU-BMI/SlicerOpenCV#30

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25181 3bd1e089-480b-0410-8dfb-8563597acbee