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

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/D29574

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@294552 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8f23b7c..8aa16e1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -131,6 +131,7 @@
 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)
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index efbea67..ed7e39e 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -61,9 +61,9 @@
   # 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)