Avoid linking libdl unless needed

[thieta]

Summary

This fixes cross-compilation when targeting a platform not using libdl and mirrors libdl linking in other parts of the code.

[mstorsjo]

I think this is ok, but adding @beanz who knows the cmake conventions better than me.

[beanz]

looks reasonable

[thieta]

@mstorsjo mind landing this for me as well?

[lebedev.ri]

FYI as reported on IRC, this likely breaks standalone build of clang

<mceier> standalone builds of clang are still supported right ? commit 0073c293a401774ac96b4b3d27f05e13f379f98e seems to break them, since HAVE_LIBDL is not defined (it requires config-ix include from llvm) causing linking libclang.so to fail with undefined reference to dladdr due to missing -ldl
<LebedevRI> assuming the versions match, sounds like a bug
<mceier> LebedevRI: by adding "-DHAVE_LIBDL -DCMAKE_DL_LIBS=dl" to cmake I was able to build standalone clang; so it is a bug

Would be good to either have a fix or a revert.

[thieta]

Thanks - I'll look into a fix.

[thieta]

@lebedev.ri fix for your issue posted here: https://reviews.llvm.org/D81385

clang/tools/libclang/CMakeLists.txt

if (TARGET clangTidyPlugin)
     if(LLVM_ENABLE_MODULES)
       list(APPEND LLVM_COMPILE_FLAGS "-fmodules-ignore-macro=CLANG_TOOL_EXTRA_BUILD")
     endif()
   else ()
     message(FATAL_ERROR "LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA needs clang-tools-extra in LLVM_ENABLE_PROJECTS")
   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)