CMake: keep derived AMReX options in sync on re-configure

[ax3l]

AMReX_PIC, ABLASTR_POSITION_INDEPENDENT_CODE and AMReX_BUILD_SHARED_LIBS were written to the cache from the "on" branch of if(ImpactX_PYTHON OR BUILD_SHARED_LIBS). Those entries outlive the requirement that created them, so re-configuring an existing build directory with -DImpactX_PYTHON=OFF silently kept the cached ON and left AMReX a shared library with no Python bindings asking for it.

Set them as normal variables instead. With CMP0077 NEW — which we already request for the ABLASTR superbuild — the downstream option() calls honor a normal variable and skip creating the cache entry, so the requirement applies for exactly the configure that asks for it and leaves nothing behind for the next one. Explicit -D values from the caller still take precedence, because they are cache entries and we no longer FORCE over them.

Details

There is deliberately no else() branch. With nothing cached there is nothing to reset, and assigning OFF there would override ABLASTR's own ABLASTR_POSITION_INDEPENDENT_CODE default of ON and change what a fresh non-Python configure builds today.

AMReX_INSTALL keeps working: ABLASTR derives it from if(DEFINED AMReX_BUILD_SHARED_LIBS), and DEFINED is true for a normal variable, so the Python configure still gets AMReX_INSTALL=ON and install(EXPORT "pyAMReXTargets" ...) is generated.

Testing

Configured ImpactX_PYTHON=ON, re-configured the same directory with ImpactX_PYTHON=OFF, and compared against a fresh ImpactX_PYTHON=OFF configure. Probed the effective variable values plus the resulting amrex_3d/ablastr_3d target TYPE and POSITION_INDEPENDENT_CODE, since the fix intentionally keeps these values out of the cache.

	fresh PYTHON=OFF	ON → re-configure OFF
development	AMReX_PIC=ON, amrex STATIC	AMReX_PIC=ON, amrex SHARED
this PR	AMReX_PIC=ON, amrex STATIC	AMReX_PIC=ON, amrex STATIC

A re-configured build directory now matches a fresh one, and the fresh configure still matches what development produces. The ImpactX_PYTHON=ON configure is unchanged from development as well: amrex SHARED + PIC, ablastr STATIC + PIC, AMReX_INSTALL=ON.

Same class of issue as BLAST-WarpX/warpx#7204 on the WarpX side.

[codspeed-hq]

Merging this PR will not alter performance

✅ 70 untouched benchmarks

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_{Comparing ax3l:cmake-cache-staleness (ba84b39) with development (ecf785f)}