[libcxxabi][CMake] Support in-tree libunwind when building as part of runtimes

[phosek]

Summary

When building as part of runtimes, there is no predefined order in which the runtimes are loaded, so the targets from other projects might not be available. We need to rely on HAVE_<name> variables instead in that case.

[beanz]

LGTM.

libcxxabi/trunk/CMakeLists.txt

 #===============================================================================
 
 # Define options.
 option(LIBCXXABI_ENABLE_EXCEPTIONS "Use exceptions." ON)
 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
+option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 option(LIBCXXABI_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)
 option(LIBCXXABI_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXXABI_HAS_EXTERNAL_THREAD_API
   "Build libc++abi with an externalized threading API.
   This option may only be set to ON when LIBCXXABI_ENABLE_THREADS=ON." OFF)
 option(LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY
   "Build libc++abi with an externalized threading library.

libcxxabi/trunk/src/CMakeLists.txt

if (LIBCXXABI_ENABLE_THREADS)
   append_if(libraries LIBCXXABI_HAS_PTHREAD_LIB pthread)
 endif()
 
 append_if(libraries LIBCXXABI_HAS_C_LIB c)
 if (LIBCXXABI_USE_LLVM_UNWINDER)
   # Prefer using the in-tree version of libunwind, either shared or static. If
   # none are found fall back to using -lunwind.
   # FIXME: Is it correct to prefer the static version of libunwind?
-  if (TARGET unwind_shared)
+  if (NOT LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_shared OR HAVE_LIBUNWIND))
     list(APPEND libraries unwind_shared)
-  elseif(TARGET unwind_static)
+  elseif (LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))
     list(APPEND libraries unwind_static)
   else()
     list(APPEND libraries unwind)
   endif()
 else()
   append_if(libraries LIBCXXABI_HAS_GCC_S_LIB gcc_s)
 endif()