diff --git a/clang/runtime/CMakeLists.txt b/clang/runtime/CMakeLists.txt
--- a/clang/runtime/CMakeLists.txt
+++ b/clang/runtime/CMakeLists.txt
@@ -95,6 +95,8 @@
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1
     USES_TERMINAL_INSTALL 1
+    # Always run the build command so that incremental builds are correct.
+    BUILD_ALWAYS 1
     )
 
   get_ext_project_build_command(run_clean_compiler_rt clean)