an environment list that can be freed using g_strfreev() (e.g., as returned from g_get_environ()), or %NULL for an empty environment list
the environment variable to set, must not contain '='
the value for to set the variable to
whether to change the variable if it already exists
Return: the updated environment list. Free it using g_strfreev().
2.32
Sets the environment variable @variable in the provided list @envp to @value.