Clear cache LZ4 variables to allow switching between builtin_lz4=ON/OFF
Clear cache LZ4 variables to allow switching between builtin_lz4=ON/OFF

If we don't clear the cache variables, CMake will keep LZ4_LIBRARY set
to $<TARGET_FILE:lz4> when switching builtin_lz4 from ON to OFF, which
makes ROOT fail to compile.

olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,karies/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root