[libcxx][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.

libcxx/trunk/CMakeLists.txt

 
 option(LIBCXX_ENABLE_ABI_LINKER_SCRIPT
       "Use and install a linker script for the given ABI library"
       ${ENABLE_LINKER_SCRIPT_DEFAULT_VALUE})
 
 # 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})
 set(LIBCXX_SYSROOT "" CACHE STRING "Use alternate sysroot.")
 set(LIBCXX_GCC_TOOLCHAIN "" CACHE STRING "Use alternate GCC toolchain.")
 
 # Feature options -------------------------------------------------------------
 option(LIBCXX_ENABLE_EXCEPTIONS "Use exceptions." ON)

libcxx/trunk/lib/CMakeLists.txt

 add_library_flags_if(LIBCXX_HAS_C_LIB c)
 add_library_flags_if(LIBCXX_HAS_M_LIB m)
 add_library_flags_if(LIBCXX_HAS_RT_LIB rt)
 add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
 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)
   endif()
 endif()
 
 # Setup flags.
 if (NOT WIN32)