diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -471,11 +471,6 @@
 endif()
 
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
-if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
-  set(LIBCXX_DEBUG_BUILD ON)
-else()
-  set(LIBCXX_DEBUG_BUILD OFF)
-endif()
 
 #===============================================================================
 # Setup Compiler Flags
@@ -585,13 +580,6 @@
   endif()
 endif()
 
-# Assertion flags =============================================================
-define_if(LIBCXX_DEBUG_BUILD -D_DEBUG)
-if (LIBCXX_ENABLE_ASSERTIONS AND NOT LIBCXX_DEBUG_BUILD)
-  # MSVC doesn't like _DEBUG on release builds. See PR 4379.
-  define_if_not(MSVC -D_DEBUG)
-endif()
-
 # Modules flags ===============================================================
 # FIXME The libc++ sources are fundamentally non-modular. They need special
 # versions of the headers in order to provide C++03 and legacy ABI definitions.
@@ -711,7 +699,7 @@
   endif()
 
   if (MSVC)
-    if (LIBCXX_DEBUG_BUILD)
+    if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
       set(LIB_SUFFIX "d")
     else()
       set(LIB_SUFFIX "")