Setting linker language for libunwind and libc++

[plotfi]

Summary

Sets the linker language to 'C' so that the driver used to link is the 'C' driver (clang) instead of the 'C++' driver (clang++). Since libc++ does not depend on an existing C++ runtime we need to avoid the implicit circular dependency to a C++ library by the driver. This means we should either use the 'C' driver (clang) instead of the 'C++' driver (clang++) or we should use the '-nostdlib++' clang (which is non-portable). Using the C driver for linking has the benefit of not longer requiring the non-portable '-nostdlib++' flag to avoid injecting the link against an existing C++ runtime.

[mgorny]

This version also breaks libc++ tests on NetBSD. Furthermore, a better explanation than 'needs to be set' would be good.

libunwind/src/CMakeLists.txt

130 ↗

(On Diff #227990)

This was already done in 6db7a5cd7c800a588e94ce5c1ef24ae4d60ecdd3.

[plotfi]

drop changed already committed by compnerd.
Move -Wl,-z,defs change to another diff.

[plotfi]

In D69874#1735019, @mgorny wrote:

This version also breaks libc++ tests on NetBSD. Furthermore, a better explanation than 'needs to be set' would be good.

Sorry about this. Hadn't thought through the full description yesterday. Updated.

[mgorny]

Actually, compnerd has reverted his change, so I suppose this is back up for discussion.

[plotfi]

In D69874#1735789, @mgorny wrote:

Actually, compnerd has reverted his change, so I suppose this is back up for discussion.

We are working on a new change to address the netbsd issues. There appears to be a case where cmake is injecting incorrect linkage when building for NetBSD, which caused the bot failures when the linker language was changed.

libcxx/src/CMakeLists.txt

if (LIBCXX_ENABLE_SHARED)
   if(COMMAND llvm_setup_rpath)
     llvm_setup_rpath(cxx_shared)
   endif()
   target_link_libraries(cxx_shared PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
+      LINKER_LANGUAGE C
       OUTPUT_NAME   "c++"
       VERSION       "${LIBCXX_ABI_VERSION}.0"
       SOVERSION     "${LIBCXX_ABI_VERSION}"
       DEFINE_SYMBOL ""
   )
   cxx_add_common_build_flags(cxx_shared)
   cxx_set_common_defines(cxx_shared)