Let the application own the MUSE_COMPILE_USE_PCH option - #254

Tracker with the merge order: audacity/audacity#11898.

Removes the MUSE_COMPILE_USE_PCH declaration from MuseDeclareOptions.cmake. The framework keeps reading the option; the application declares it, like MUSE_COMPILE_USE_UNITY, and no longer has to declare it first to control the default (audacity/audacity#11795).

An application that does not declare it builds without PCH, silently. audacity/audacity#11900 and musescore/MuseScore#34791 (one line each) must merge before each application bumps its muse submodule past this PR. This PR itself can merge before or after them.

The standalone CMakeLists.txt declares both options OFF, so a standalone framework build is strict by default (possible since #256). The unit-test job still forces unity ON in ninja_build_utests.sh; the CI net for the fully strict configuration is audacity/audacity#12246.

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Build configuration

audacity: audacity/audacity/master
audacity platforms: linux_x64
musescore: musescore/MuseScore/main
musescore platforms: linux_x64