According to the CMake documentation
https://cmake.org/cmake/help/v3.3/command/set.html
One can do
set(ENV{<variable>} <value>)
but this gives the result
set(ENV{FOO} foo)
message("variable is $ENV{FOO}")
at configure time
variable is foo
But at Linux command
echo $FOO
the variable is not set.
EDIT:
Here's a partial solution to the problem, which was to set $PATH, so that a user has CMAKE_INSTALL_PREFIX listed first
set(file_sh ${CMAKE_CURRENT_BINARY_DIR}/path.sh)
set(path "${CMAKE_INSTALL_PREFIX}:$ENV{PATH}")
file(WRITE ${file_sh} "#!/usr/bin/env bash\n")
file(APPEND ${file_sh} "export PATH=\"${path}\"")
execute_process(COMMAND chmod a+x ${file_sh} RESULT_VARIABLE res)
this creates this file
#!/usr/bin/env bash
export PATH="/install/prefix/path:/other/path"
that later can be executed on a bash terminal with
source path.sh