[CMake] Improve Clang directory deduction in core/clingutils

Assume that the Clang versions alsways matches the LLVM version to
simplify the code, and also make it possible to find Clang installations
separate from the LLVM directory by considering `CLANG_INSTALL_PREFIX`.

Author: guitargeek

core/clingutils/CMakeLists.txt

@@ -80,31 +80,15 @@ foreach(dict ${stldicts})
   target_include_directories(${dict}Dict PRIVATE ${CMAKE_SOURCE_DIR}/interpreter/cling/include/cling/cint)
 endforeach()
 
-set(CLANG_RESOURCE_DIR_STEM)
-if (builtin_clang)
-  set(CLANG_RESOURCE_DIR_STEM ${CMAKE_BINARY_DIR}/interpreter/llvm-project/llvm/${CMAKE_CFG_INTDIR}/lib/clang)
-  set(CLANG_RESOURCE_DIR_VERSION ${LLVM_VERSION_MAJOR})
-else ()
-  set(CLANG_RESOURCE_DIR_STEM ${LLVM_LIBRARY_DIR}/clang)
-  # A user can define a clang version to use, otherwise find it (but will error if more than one version is present)
-  if (NOT DEFINED CLANG_RESOURCE_DIR_VERSION)
-    if (NOT EXISTS ${CLANG_RESOURCE_DIR_STEM})
-      message(FATAL_ERROR "${CLANG_RESOURCE_DIR_STEM} does not exist. Please install clang.")
-    endif()
-    # There is no reasonable way to get the version of clang under which is its resource directory.
-    # For example, lib/clang/5.0.0/include. Deduce it.
-    file(GLOB CHILDREN RELATIVE ${CLANG_RESOURCE_DIR_STEM} ${CLANG_RESOURCE_DIR_STEM}/*)
-    list(LENGTH CHILDREN CHILDREN_LENGTH)
-    if (${CHILDREN_LENGTH} GREATER 1)
-      message(FATAL_ERROR "Found more than one version of clang. CLANG_RESOURCE_DIR_VERSION contains: '${CHILDREN}'." )
-    endif()
-
-    list(GET CHILDREN 0 CLANG_RESOURCE_DIR_VERSION)
-  endif()
+# If no explicit Clang prefix is set, assume it lives inside LLVM prefix.
+if(NOT DEFINED CLANG_INSTALL_PREFIX)
+  set(CLANG_INSTALL_PREFIX ${LLVM_DIR})
 endif()
 
+# We only look for the Clang version that matches the LLVM version, because other setups are not supported.
+set(CLANG_RESOURCE_DIR_VERSION ${LLVM_VERSION_MAJOR})
 
-set(CLANG_RESOURCE_DIR ${CLANG_RESOURCE_DIR_STEM}/${CLANG_RESOURCE_DIR_VERSION}/include)
+set(CLANG_RESOURCE_DIR ${CLANG_INSTALL_PREFIX}/lib/clang/${CLANG_RESOURCE_DIR_VERSION}/include)
 
 #---Deal with clang resource here----------------------------------------------
 install(DIRECTORY ${CMAKE_BINARY_DIR}/etc/cling/lib/clang/${CLANG_RESOURCE_DIR_VERSION}/include/