[libcxxabi] [cmake] Handle missing LIBUNWIND_* directories gracefully

[mgorny]

Summary

Add LIBUNWIND_* directories to include path only if they were actually
found, in order to fix the CMake error. Both of the directories are
usually unnecessary since libcxxabi uses only the common part of
unwind.h that is supplied both by GCC and Clang.

[mgorny]

Ping.

[mgorny]

Second ping.

[jroelofs]

LGTM

libcxxabi/trunk/CMakeLists.txt

find_path(
     NO_DEFAULT_PATH
   )
 
   if (LIBCXXABI_LIBUNWIND_SOURCES STREQUAL "LIBCXXABI_LIBUNWIND_SOURCES-NOTFOUND")
     message(WARNING "LIBCXXABI_LIBUNWIND_SOURCES was not specified and couldn't be infered.")
     set(LIBCXXABI_LIBUNWIND_SOURCES "")
   endif()
 
+  if (NOT LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL STREQUAL "LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL-NOTFOUND")
     include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL}")
+  endif()
+  if (NOT LIBCXXABI_LIBUNWIND_SOURCES STREQUAL "")
     include_directories("${LIBCXXABI_LIBUNWIND_SOURCES}")
   endif()
+endif()
 
 # Add source code. This also contains all of the logic for deciding linker flags
 # soname, etc...
 add_subdirectory(src)
 
 if (NOT LIBCXXABI_INCLUDE_TESTS OR (LIBCXXABI_STANDALONE_BUILD AND NOT LIBCXXABI_ENABLE_SHARED))
   # We can't reasonably test the system C++ library with a static libc++abi.
   # We either need to be able to replace libc++abi at run time (with a shared