diff --git a/lib/clang/libllvmsupport/Makefile b/lib/clang/libllvmsupport/Makefile
index 1a2dd20c574c..6dff622422b8 100644
--- a/lib/clang/libllvmsupport/Makefile
+++ b/lib/clang/libllvmsupport/Makefile
@@ -93,3 +93,8 @@ SRCS+=	DataExtractor.cpp \
 .endif
 
 .include "../clang.lib.mk"
+
+# Ugly hack to work around CLOCK_PROCESS_CPUTIME_ID not being properly defined
+# between r239347 and r245428.
+CXXFLAGS.Process.cpp=	-DCLOCK_PROCESS_CPUTIME_ID=15
+CXXFLAGS+=		${CXXFLAGS.${.IMPSRC:T}}