Stop rebuilding Proxygen StatsWrapper

[mavam]

🔍 Problem

• The bundled Proxygen StatsWrapper.h workaround uses an always-dirty custom target.
• Ninja regenerates the header on every incremental build, even when inputs did not change.

🛠️ Solution

• Generate the build-tree copy during CMake configuration instead.
• Keep the generated header present before Ninja evaluates Proxygen's generated-file rule.
• Remove the custom target that forced repeated regeneration.

💬 Review

• Focus on whether configure-time generation is the least invasive workaround for the vendored Proxygen rule.
• Verified with repeated cmake --build --preset macos-xcode-release; the second build no longer regenerates StatsWrapper.h.

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[tobim]

Please don't invest so much time into this workaround. The issue is upstream.

You can open a PR there to change the custom command to

add_custom_command(
    OUTPUT ${PROXYGEN_GENERATED_ROOT}/proxygen/lib/stats/StatsWrapper.h
    COMMAND
        ${CMAKE_COMMAND} -E make_directory
        ${PROXYGEN_GENERATED_ROOT}/proxygen/lib/stats
    COMMAND
        ${CMAKE_CURRENT_SOURCE_DIR}/stats/gen_StatsWrapper.sh
        ${PROXYGEN_GENERATED_ROOT}
    DEPENDS
        ${CMAKE_CURRENT_SOURCE_DIR}/stats/gen_StatsWrapper.sh
        ${CMAKE_CURRENT_SOURCE_DIR}/stats/BaseStats.h
    COMMENT "Generating StatsWrapper.h"
    VERBATIM
)

until that is available we can accept the few milliseconds the generator takes.
Once that is fixed we can drop the entire statswrapper workaround.
Edit: the code to change is in: https://github.com/facebook/proxygen/blob/main/proxygen/lib/CMakeLists.txt#L30

[mavam]

Closing this in favor of an upstream Proxygen fix: facebook/proxygen#614

The local workaround became too involved for the small incremental-build cost. Once upstream is fixed and the vendored Proxygen is updated, we can drop the existing StatsWrapper workaround entirely.