[CMake] Fix c++ abi library configuration on Linux.

You can now configure from the command line using:
-DLIBCXX_CXX_ABI=libsupc++
-DLIBCXX_LIBSUPCXX_INCLUDE_PATHS="path;path

Also documents how to build on Linux.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@171316 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CMakeLists.txt

@@ -31,7 +31,9 @@ else()
     )
 endif()
 
-add_dependencies(cxx ${LIBCXX_CXX_ABI_DEPS})
+if (DEFINED LIBCXX_CXX_ABI_DEPS)
+  add_dependencies(cxx ${LIBCXX_CXX_ABI_DEPS})
+endif()
 
 # Generate library list.
 set(libraries ${LIBCXX_CXX_ABI_LIBRARIES})