[CMake] [CPACK] Add and define an environment variable for NSIS

[Olivier Pierard]

Dear all,

I would like to add a step for our NSIS installer in order to define an
environment variable to locate a license file which is sent separately
to the package (best would be locating it through a 'browse' button).

I'm a little bit lost in the CPack files organization so if someone
could give me some hints on the best practice to do so, I would be very
grateful.  Should I use the CPACK_EXTRA_INSTALL_COMMAND ?  Should I copy
and modify NSIS.InstallOptions.ini.in and/or NSIS.template.in ?

I'm using cmake 2.8.0 (but can upgrade it if needed) and this should be
valid for any Windows platform.

Thanks for your help,

Olivier