[interpreter] Suppress STATUS messages from LLVM

LLVM prints which projects and targets are enabled on every CMake
invocation. This information is noise for most users of ROOT, so
turn it off unless the user explicitly decided to see it.

interpreter/CMakeLists.txt

@@ -218,13 +218,23 @@ if(builtin_llvm)
 
   set(LLVM_ENABLE_PROJECTS "clang" CACHE STRING "")
 
+  #---Reduce log level to suppress STATUS messages from LLVM
+  if(NOT DEFINED CMAKE_MESSAGE_LOG_LEVEL)
+    set(CMAKE_MESSAGE_LOG_LEVEL "NOTICE")
+    set(_unset_log_level TRUE)
+  endif()
+
   #---Add the sub-directory excluding all the targets from all-----------------------------------------
   if(CMAKE_GENERATOR MATCHES "Xcode")
     add_subdirectory(llvm-project/llvm)
   else()
     add_subdirectory(llvm-project/llvm EXCLUDE_FROM_ALL)
   endif()
 
+  if(DEFINED _unset_log_level)
+    unset(CMAKE_MESSAGE_LOG_LEVEL)
+  endif()
+
   set(LLVM_INCLUDE_DIRS ${CMAKE_SOURCE_DIR}/interpreter/llvm-project/llvm/include
     ${CMAKE_BINARY_DIR}/interpreter/llvm-project/llvm/include
     CACHE STRING "LLVM include directories."