[libc++][CMake] Use debug MSVC runtimes when libc++ is built in debug mode

Summary: This patch allows libc++ to be built against the debug MSVC runtimes instead of just the release ones.

Reviewers: rnk, majnemer, compnerd, smeenai

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D28725

llvm-svn: 292006
diff --git a/libcxx/test/CMakeLists.txt b/libcxx/test/CMakeLists.txt
index ad110fb..b9d5f91 100644
--- a/libcxx/test/CMakeLists.txt
+++ b/libcxx/test/CMakeLists.txt
@@ -35,6 +35,7 @@
 pythonize_bool(LIBCXX_HAS_ATOMIC_LIB)
 pythonize_bool(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
 pythonize_bool(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
+pythonize_bool(LIBCXX_DEBUG_BUILD)
 
 # By default, for non-standalone builds, libcxx and libcxxabi share a library
 # directory.