Bug 315061 – [CMake] Avoid unnecessary rebuilds after CMake preset reconfigure

• Status: RESOLVED
• Resolution: FIXED
• Component: CMake

[zak ridouh]

Several CMake helpers wrote generated build inputs (the ccache
launcher, a Swift modulemap, a WebPushDaemon stub, an empty WebGPU
cmakeconfig.h, a LayoutTestHelper stub config.h, an AppleFeatures
stub) using file(WRITE), which is unconditional and always bumps the
mtime. The rewrite-compile-commands script did the same. As a result,
every reconfigure marked a chunk of PCHs and generated sources stale
even when their content was identical, forcing ~70 build steps and a
framework relink.

Switch those writes to file(CONFIGURE OUTPUT ... CONTENT ...) and
teach rewrite-compile-commands to compare before writing. Both only
update the file when content actually differs, so mtimes stay stable
across no-op reconfigures.

This drops the no changes reconfigure + rebuild time from ~50s to ~1s.

[Radar WebKit Bug Importer]

<rdar://problem/177387522>

[zak ridouh]

Pull request: https://github.com/WebKit/WebKit/pull/65150

[EWS]

Committed 313460@main (9da1138b4ab4): <https://commits.webkit.org/313460@main>

Reviewed commits have been landed. Closing PR #65150 and removing active labels.