Fix libdl linking for libclang in standalone mode

[thieta]

Summary

As noted in https://reviews.llvm.org/D80492 that change broke standalone builds of clang. This reverts the old behavior of libdl linking when in standalone mode.

As a note I think it would be safe to just do:

list(APPEND LIBS ${CMAKE_DL_LIBS})

But I opted for the pragmatic fix this time. Let me know if you think we should try that instead.

[lebedev.ri]

Thank you!

<LebedevRI> mceier: https://reviews.llvm.org/D81385
<mceier> LebedevRI: clang built successfully with that patch

But consider waiting for @beanz

[mgorny]

Ping. It'd be nice to have this fixed.

[mstorsjo]

Ping @beanz - I think people would like this to be included before the 11 branch.

[mstorsjo]

Ping @beanz

@hans - I think this is something that would be wanted to have fixed in the release branch (it's a regression for certain build configurations, afaik) - the fix awaits an ack from @beanz.

[hans]

In D81385#2162911, @mstorsjo wrote:

Ping @beanz

@hans - I think this is something that would be wanted to have fixed in the release branch (it's a regression for certain build configurations, afaik) - the fix awaits an ack from @beanz.

Thanks, I've put it on my list. It needs to land first though :)

[lebedev.ri]

In D81385#2164235, @hans wrote:

In D81385#2162911, @mstorsjo wrote:

Ping @beanz

@hans - I think this is something that would be wanted to have fixed in the release branch (it's a regression for certain build configurations, afaik) - the fix awaits an ack from @beanz.

Thanks, I've put it on my list. It needs to land first though :)

I think that was enough of a headroom to provide feedback.
Please proceed to commit, there's always post-commit review.

[hans]

Pushed to 11.x as 833f8c958601bb640ba6a25d627c1dc58dad14d2. Please let me know if there are any follow-ups.

clang/tools/libclang/CMakeLists.txt

if (TARGET clangTidyPlugin)
     endif()
   else ()
     message(FATAL_ERROR "LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA needs clang-tools-extra in LLVM_ENABLE_PROJECTS")
   endif ()
 endif ()
 
 if (HAVE_LIBDL)
   list(APPEND LIBS ${CMAKE_DL_LIBS})
-endif()
+elseif (CLANG_BUILT_STANDALONE)
+  find_library(DL_LIBRARY_PATH dl)
+  if (DL_LIBRARY_PATH)
+    list(APPEND LIBS dl)
+  endif ()
+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)
   # Avoid LNK4197 by not specifying libclang.exports here.