This boolean setting allows parallel building to be disabled for
individual source files built via `add_custom_command`. Using this
option is equivalent to setting policy `CMP0147` to the `OLD` behavior.
Closes: #26413
Every policy's documentation has a paragraph on what version of CMake
introduced it, how to set the policy, and whether CMake warns if the
policy is not set. The wording of this paragraph has diverged across
policies over time. Factor the paragraph out into a standard advice
document included by every policy.
In commit 33c15ae2b9 (VS: Build custom commands concurrently when
possible, 2023-01-19, v3.26.0-rc1~56^2) we added `BuildInParallel` to
custom commands in `.vcxproj` files, but that had to be reverted by
commit abb1c12162 (VS: Revert "Build custom commands concurrently when
possible", 2023-03-07, v3.26.0-rc6~3^2) because some projects may have
custom commands that accidentally rely on serial execution in MSBuild.
Add a policy to use `BuildInParallel` for custom commands in projects
that have been updated to set the policy to `NEW`.
Fixes: #18405
Darragh Coy