Detect libdl from cmake variable

[raj.khem]

Summary

This helps in getting right settings for libdl irrespective of platform
it also fixes build with -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_LINK_LLVM_DYLIB=ON

Fixes

tools/clang/tools/libclang/CMakeFiles/libclang.dir/CIndexer.cpp.o: In function clang::CIndexer::getClangResourcesPath[abi:cxx11]()': 2019-12-06T06:04:39.2987601Z | CIndexer.cpp:(.text._ZN5clang8CIndexer21getClangResourcesPathB5cxx11Ev+0x8b): undefined reference to dladdr'

[raj.khem]

0073c293a401774ac96b4b3d27f05e13f379f98e has fixed it.

clang/tools/libclang/CMakeLists.txt

if (TARGET clangTidyPlugin)
   add_definitions(-DCLANG_TOOL_EXTRA_BUILD)
   list(APPEND LIBS clangTidyPlugin)
   list(APPEND LIBS clangIncludeFixerPlugin)
   if(LLVM_ENABLE_MODULES)
     list(APPEND LLVM_COMPILE_FLAGS "-fmodules-ignore-macro=CLANG_TOOL_EXTRA_BUILD")
   endif()
 endif ()
 
-find_library(DL_LIBRARY_PATH dl)
-if (DL_LIBRARY_PATH)
-  list(APPEND LIBS dl)
+if (HAVE_LIBDL)
+  list(APPEND LIBS ${CMAKE_DL_LIBS})
 endif()
 
 option(LIBCLANG_BUILD_STATIC
   "Build libclang as a static library (in addition to a shared one)" OFF)
 
 set(LLVM_EXPORTED_SYMBOL_FILE ${CMAKE_CURRENT_SOURCE_DIR}/libclang.exports)
 
 if(MSVC)