Disable ccache for libxc's targets

[ryanmrichard]

Summary

• A ccache entry for one of libxc's object files was found corrupted/truncated (most likely from a prior cancelled or OOM-killed CI run) and served as a false hit: it built and linked cleanly but crashed with SIGILL at runtime deep inside a libxc functional evaluation (test_unit_scf/test_integration_scf's libxc SVWN3 tests, seen intermittently on NWChemEx/SCF#70's test_cmake_build (ubuntu-latest, gcc-14) leg).
• Reproduced and confirmed: a from-scratch build (no ccache) never crashed; manually deleting the ccache-Linux-gcc-14-* GitHub Actions cache and re-running fixed the real PR job. Root cause is a bad cache entry, not libxc's source, GCC, or CPU/ISA.
• The corrupted entry has since been cleared manually, but the underlying risk remains: ccache is reused across CI runs (and potentially different physical hosts) via a restore-key fallback, so this class of bug can recur silently.
• Fix: clear C_COMPILER_LAUNCHER/CXX_COMPILER_LAUNCHER on libxc's targets (xc, and xcf03 if Fortran bindings are enabled) right after FetchContent_MakeAvailable(libxc), so this one dependency is never routed through ccache. Its own build is a few seconds, and FetchContent already caches the source checkout separately, so the object-level reuse given up here is cheap insurance.

Test plan

• Minimal local repro confirming set_target_properties(<tgt> PROPERTIES C_COMPILER_LAUNCHER "") after add_library correctly drops the LAUNCHER from that target's Ninja compile rule while a sibling target keeps the global launcher.
• Once merged, SCF PR Fix broken FetchContent: point chemcache at master #70 should no longer be able to hit this SIGILL class of failure regardless of ccache state.

🤖 Generated with Claude Code

[github-actions]

🚀 [bumpr] Bumped!
New version:v0.0.20
Changes:v0.0.19...v0.0.20