Add a CPACK_NSIS_EXECUTABLE cpack var to be able to choose makensis.exe location
See here: https://discourse.cmake.org/t/how-to-select-nsis-on-a-non-standard-location/2570
Currently, to chooose NSIS location, modifying the PATH env var is needed, this is not practical.
According to this page, a patch already exist somewhere: https://wiki.vcmi.eu/How_to_build_VCMI_(Linux/Cmake/MXE)
Edited by Mathieu Westphal (Kitware)