re-add _DEBUG for Debug builds, and print undefines in verbose mode

Author: nickdesaulniers

libc/CMakeLists.txt

@@ -7,6 +7,12 @@ endif()
 include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
   NO_POLICY_SCOPE)
 
+if (LIBC_CMAKE_VERBOSE_LOGGING)
+  get_directory_property(LIBC_OLD_PREPROCESSOR_DEFS COMPILE_DEFINITIONS)
+  foreach(OLD_DEF ${LIBC_OLD_PREPROCESSOR_DEFS})
+    message(STATUS "Undefining ${OLD_DEF}")
+  endforeach()
+endif()
 set_directory_properties(PROPERTIES
   # `llvm-project/llvm/CMakeLists.txt` adds the following directive
   # `include_directories( ${LLVM_INCLUDE_DIR} ${LLVM_MAIN_INCLUDE_DIR})` We
@@ -16,6 +22,9 @@ set_directory_properties(PROPERTIES
   # to set a few preprocessor defines which we do not want.
   COMPILE_DEFINITIONS ""
 )
+if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+  add_definitions("-D_DEBUG")
+endif()
 
 # Default to C++17
 set(CMAKE_CXX_STANDARD 17)