[libcxx][CMake] Support in-tree libunwind when building as part of ru…

…ntimes

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.

Differential Revision: https://reviews.llvm.org/D29575

llvm-svn: 294553

libcxx/CMakeLists.txt

@@ -158,6 +158,7 @@ option(LIBCXX_ENABLE_ABI_LINKER_SCRIPT
 # Build libc++abi with libunwind. We need this option to determine whether to
 # link with libunwind or libgcc_s while running the test cases.
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
+option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++." ${LLVM_BUILD_32_BITS})

libcxx/lib/CMakeLists.txt

@@ -89,9 +89,9 @@ add_library_flags_if(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB atomic)
 
 # Add the unwinder library.
 if (LIBCXXABI_USE_LLVM_UNWINDER)
-  if (TARGET unwind_shared)
+  if (NOT LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_shared OR HAVE_LIBUNWIND))
     add_interface_library(unwind_shared)
-  elseif (TARGET unwind_static)
+  elseif (LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))
     add_interface_library(unwind_static)
   else()
     add_interface_library(unwind)