[CMAKE] Always build obj/build.h
[CMAKE] Always build obj/build.h

Summary:
The file needs to be always generated.
This requires the use of a `custom_target` (always out of date) rather
than a `custom_command`.

Test Plan:
  mkdir buildcmake && cd buildcmake
  cmake -GNinja ..
  ninja
Check the `buildcmake/obj/build.h` file content matches your current
commit hash.
Amend the commit message then run `ninja` again: the
`buildcmake/obj/build.h` should be updated accordingly.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4111

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