Fix finding libunwind when only using LIBCXXABI_USE_LLVM_UNWINDER=ON

[wjywbs]

Summary

LIBCXXABI_LIBUNWIND_INCLUDES is already set before calling find_path, and find_path will not update it. So cmake will throw error "include_directories given empty-string as include directory". Use other variable name for the include path and do not find the default libunwind.h in the system path.

[asl]

LGTM

[wjywbs]

Could you please commit for me because I am not a committer? Thanks.

[asl]

http://reviews.llvm.org/rL236936

CMakeLists.txt

 set(LIBCXXABI_LIBUNWIND_INCLUDES "${LIBCXXABI_LIBUNWIND_INCLUDES}" CACHE PATH
     "Specify path to libunwind includes." FORCE)
 set(LIBCXXABI_LIBUNWIND_PATH "${LIBCXXABI_LIBUNWIND_PATH}" CACHE PATH
     "Specify path to libunwind source." FORCE)
 
 include_directories(include)
 if (LIBCXXABI_USE_LLVM_UNWINDER OR LLVM_NATIVE_ARCH MATCHES ARM)
   find_path(
-    LIBCXXABI_LIBUNWIND_INCLUDES
+    LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL
     libunwind.h
     PATHS ${LIBCXXABI_LIBUNWIND_INCLUDES}
           ${LIBCXXABI_LIBUNWIND_PATH}/include
           ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBUNWIND_INCLUDES}
           ${LLVM_MAIN_SRC_DIR}/projects/libunwind/include
+    NO_DEFAULT_PATH
   )
 
   find_path(
     LIBCXXABI_LIBUNWIND_SOURCES
     libunwind_ext.h
     PATHS ${LIBCXXABI_LIBUNWIND_PATH}/src/
           ${LIBCXXABI_LIBUNWIND_INCLUDES}/../src/
           ${LLVM_MAIN_SRC_DIR}/projects/libunwind/src/
     NO_DEFAULT_PATH
   )
 
   if (LIBCXXABI_LIBUNWIND_SOURCES STREQUAL "LIBCXXABI_LIBUNWIND_SOURCES-NOTFOUND")
     message(WARNING "LIBCXXABI_LIBCXX_PATH was not specified and couldn't be infered.")
     set(LIBCXXABI_LIBUNWIND_SOURCES "")
   endif()
 
-  include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES}")
+  include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL}")
   include_directories("${LIBCXXABI_LIBUNWIND_SOURCES}")
 endif ()
 
 # Add source code. This also contains all of the logic for deciding linker flags
 # soname, etc...
 add_subdirectory(src)
 
 if(NOT LIBCXXABI_ENABLE_SHARED)