How to modify environment variables passed to custom CMake target?

A portable way of setting environment variables for a custom target is to use CMake's command-line tool mode command env:

env [--unset=NAME]... [NAME=VALUE]... COMMAND [ARG]...

Run command in a modified environment.

E.g.:

add_custom_target(newtarget ${CMAKE_COMMAND} -E env NAME=VALUE somecommand)

Also see Command Line Tool Mode.

You set environment variable at configuration step, but command specified for add_custom_target is executed at build step. See also CMake FAQ: How can I get or set environment variables?

[...]
environment variables SET in the CMakeLists.txt only take effect for cmake itself (configure-time), so you cannot use this method to set an environment variable that a custom command might need (build-time). Barring environment variable support by various CMake commands (e.g. add_custom_command(), currently not supported yet), an acceptable workaround may be to invoke shell scripts instead which wrap the commands to be executed.

Currently add_custom_target (and others commands, which define actions for build step, e.g. add_custom_command) doesn't support simple setting environment variables. As adviced in this bugreport, for set variable's value without spaces on Linux you may prepend command with "VAR=VAL" clauses. For general cases you may prepare wrapper script, which setups environment and run actual command:

On Windows:

wrapper.bat:

@ECHO OFF
set PATH=C:\\Some\\Path;%PATH%
%*

CMakeLists.txt:

add_custom_target(...
    COMMAND cmd /c ${CMAKE_CURRENT_SOURCE_DIR}/wrapper.bat <real_command> args...
)

On Linux:

wrapper.sh:

export "PATH=/Some/Path:$PATH"
eval "$*"

CMakeLists.txt:

add_custom_target(...
    COMMAND /bin/sh ${CMAKE_CURRENT_SOURCE_DIR}/wrapper.sh <real_command> args...
)

If value of variable depends on configuration, you may configure wrapper script with configure_file.

UPDATE:

As noted by @sakra, env tool mode of cmake executable can be used as a wrapper script:

add_custom_target(...
    COMMAND ${CMAKE_COMMAND} -E env "PATH=C:/Some/Path;$ENV{PATH}" <real_command> args...
)

This way is available since CMake 3.2.