CMake: Fix `FindClang` not finding system clang on linux in some cases.

In Debian e.g. Clang is part of LLVM, so we need to also check its root
directory sometimes to find Clang files.

build_files/cmake/Modules/FindClang.cmake

@@ -20,8 +20,24 @@ if(NOT CLANG_ROOT_DIR AND NOT $ENV{CLANG_ROOT_DIR} STREQUAL "")
   set(CLANG_ROOT_DIR $ENV{CLANG_ROOT_DIR})
 endif()
 
+if(NOT LLVM_ROOT_DIR)
+  if(DEFINED LLVM_VERSION)
+    message(running llvm-config-${LLVM_VERSION})
+    find_program(LLVM_CONFIG llvm-config-${LLVM_VERSION})
+  endif()
+  if(NOT LLVM_CONFIG)
+    find_program(LLVM_CONFIG llvm-config)
+  endif()
+
+  execute_process(COMMAND ${LLVM_CONFIG} --prefix
+          OUTPUT_VARIABLE LLVM_ROOT_DIR
+          OUTPUT_STRIP_TRAILING_WHITESPACE)
+  set(LLVM_ROOT_DIR ${LLVM_ROOT_DIR} CACHE PATH "Path to the LLVM installation")
+endif()
+
 set(_CLANG_SEARCH_DIRS
   ${CLANG_ROOT_DIR}
+  ${LLVM_ROOT_DIR}
   /opt/lib/clang
 )