[cmake] Add variables for extra hints where the clang and llvm config…

…s are.

This allows us to control externally the find_package machinery to discover
non-canonical setups such as the one in the ROOT project when using external
llvm and builtin clang.

CMakeLists.txt

@@ -63,7 +63,7 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   endif()
 
   if (NOT LLVM_FOUND AND DEFINED LLVM_DIR)
-    find_package(LLVM REQUIRED CONFIG PATHS ${LLVM_DIR} "${LLVM_DIR}/lib/cmake/llvm" "${LLVM_DIR}/cmake" NO_DEFAULT_PATH)
+    find_package(LLVM REQUIRED CONFIG PATHS ${LLVM_DIR} "${LLVM_DIR}/lib/cmake/llvm" "${LLVM_DIR}/cmake" "${LLVM_CONFIG_EXTRA_PATH_HINTS}" NO_DEFAULT_PATH)
   endif()
 
   if (NOT LLVM_FOUND)
@@ -99,7 +99,7 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   endif()
 
   if (NOT Clang_FOUND AND DEFINED Clang_DIR)
-    find_package(Clang REQUIRED CONFIG PATHS ${Clang_DIR} "${Clang_DIR}/lib/cmake/clang" "${Clang_DIR}/cmake" NO_DEFAULT_PATH)
+    find_package(Clang REQUIRED CONFIG PATHS ${Clang_DIR} "${Clang_DIR}/lib/cmake/clang" "${Clang_DIR}/cmake" "${Clang_CONFIG_EXTRA_PATH_HINTS}" NO_DEFAULT_PATH)
   endif()
 
   if (NOT Clang_FOUND)