Avoid reconfiguring llvm on a source change.

This reimplements root-project/root@13e0c197f3

interpreter/CMakeLists.txt

@@ -4,6 +4,7 @@ if (NOT builtin_clang)
 endif()
 
 #---Define the way we want to build and what of llvm/clang/cling------------------------------------
+set(LLVM_APPEND_VC_REV OFF CACHE BOOL "")
 set(LLVM_ENABLE_WARNINGS OFF CACHE BOOL "")
 set(LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
 set(LLVM_INCLUDE_BENCHMARKS OFF CACHE BOOL "")