[CMake] Get Clang directory from CLANG_INSTALL_PREFIX

If Clang was installed standalone (in particular, not in the LLVM tree),
use the CLANG_INSTALL_PREFIX variable to indicate where it is.

Actually, we can always use `CLANG_INSTALL_PREFIX`. It's defined by
`ClangConfig.cmake`. To make sure that this works, `find_package(Clang)`
has to be called in `core/clingutils/CMakeLists.txt`.

Author: guitargeek

core/clingutils/CMakeLists.txt

@@ -85,7 +85,7 @@ if (builtin_clang)
   set(CLANG_RESOURCE_DIR_STEM ${CMAKE_BINARY_DIR}/interpreter/llvm-project/llvm/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX}/clang)
   set(CLANG_RESOURCE_DIR_VERSION ${LLVM_VERSION_MAJOR})
 else ()
-  set(CLANG_RESOURCE_DIR_STEM ${LLVM_LIBRARY_DIR}/clang)
+  set(CLANG_RESOURCE_DIR_STEM ${CLANG_INSTALL_PREFIX}/lib${LLVM_LIBDIR_SUFFIX}/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})

interpreter/CMakeLists.txt

@@ -55,8 +55,10 @@ set(LLVM_FORCE_USE_OLD_TOOLCHAIN ON CACHE BOOL "")
 # (Note that the option is very counter-intuitive: We turn *on* disabling it...)
 set(CMAKE_DISABLE_FIND_PACKAGE_CUDA ON)
 
-# will be set again in case NOT builtin_llvm
-set(LLVM_DIR "${CMAKE_CURRENT_BINARY_DIR}/llvm-project/llvm")
+if(builtin_llvm)
+  set(LLVM_DIR "${CMAKE_CURRENT_BINARY_DIR}/llvm-project/llvm")
+endif()
+
 if (clad)
   set(CLING_BUILD_PLUGINS ON)
 endif()
@@ -408,6 +410,9 @@ if (builtin_clang)
 else()
   find_package(Clang REQUIRED CONFIG)
   message(STATUS "Found Clang ${CLANG_PACKAGE_VERSION} in ${CLANG_CMAKE_DIR}")
+
+  # Forward CLANG_INSTALL_PREFIX so it can be used by core/clingutils/CMakeLists.txt
+  set(CLANG_INSTALL_PREFIX ${CLANG_INSTALL_PREFIX} PARENT_SCOPE)
 endif()
 
 # Reset the compiler flags after compiling LLVM and Clang