cmake: Add mp_headers custom target

[hebasto]

This convenience target allows generating Cap'n Proto C++ source files, which is useful as the capnp_generate_cpp function is not CODEGEN-aware.

Required for bitcoin/bitcoin#35468.

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Stale ACK	ryanofsky

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[ryanofsky]

Code review ACK 941f692. It seems like a good idea to expose a target that will generate the library headers so outside targets can depend on them, and since they are probably needed for most static analysis.

But I'm not sure why PR description says this is required for bitcoin/bitcoin#35468 when it doesn't seem to be referenced there. I wonder if this target is required for anything in particular.

Could also consider making this definition more consistent with target_capnp_sources:

libmultiprocess/cmake/TargetCapnpSources.cmake

Line 118 in 61de697

add_custom_target("${target}_headers" DEPENDS ${generated_headers})

with:

add_custom_target(mp_headers DEPENDS ${MP_PROXY_HDRS})

But current definition seems ok.

[hebasto]

But I'm not sure why PR description says this is required for bitcoin/bitcoin#35468 when it doesn't seem to be referenced there. I wonder if this target is required for anything in particular.

I've updated bitcoin/bitcoin#35468. Please see bitcoin/bitcoin#35468 (comment).

[hebasto]

Could also consider making this definition more consistent with target_capnp_sources:

libmultiprocess/cmake/TargetCapnpSources.cmake

Line 118 in 61de697

add_custom_target("${target}_headers" DEPENDS ${generated_headers})

with:

add_custom_target(mp_headers DEPENDS ${MP_PROXY_HDRS})

I've been thinking about changes in the opposite direction. The _codegen suffix for such build targets seems consistent with CMake's codegen default target.