CMP0175

add_custom_command() rejects invalid arguments.

CMake 3.30 and earlier silently ignored unsupported keywords and missing or invalid arguments for the different forms of the add_custom_command() command. CMake 3.31 implements more rigorous argument checking and will flag invalid or missing arguments as errors.

The OLD behavior of this policy will accept the same invalid keywords or arguments as CMake 3.30 and earlier. The NEW behavior will flag the following as errors that previously went unreported:

• The OUTPUT form does not accept PRE_BUILD, PRE_LINK, or POST_BUILD keywords.
• When the APPEND keyword is given, the OUTPUT form also does not accept BYPRODUCTS, COMMAND_EXPAND_LISTS, DEPENDS_EXPLICIT_ONLY, DEPFILE, JOB_POOL, JOB_SERVER_AWARE, USES_TERMINAL, or VERBATIM keywords.
• The TARGET form requires exactly one of PRE_BUILD, PRE_LINK, or POST_BUILD to be given. Previously, if none were given, POST_BUILD was assumed, or if multiple keywords were given, the last one was used.
• The TARGET form does not accept DEPENDS, DEPENDS_EXPLICIT_ONLY, DEPFILE, IMPLICIT_DEPENDS, MAIN_DEPENDENCY, JOB_POOL, or JOB_SERVER_AWARE keywords.
• The TARGET form now requires at least one COMMAND to be given.
• If a keyword expects a value to be given after it, but no value is provided, that was previously treated as though the keyword was not given at all.
• The COMMENT keyword expects exactly one value after it. If multiple values are given, or if the COMMENT keyword is given more than once, this is an error.

This policy was introduced in CMake version 3.31. It may be set by cmake_policy() or cmake_minimum_required(). If it is not set, CMake warns, and uses OLD behavior.