diff --git a/Makefile.am b/Makefile.am
index 0f7a04ea..5bdc3093 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -54,7 +54,7 @@ endif
 
 # Set _LINUX_SOURCE_COMPAT for AIX for malloc(0).
 if IS_AIX
-DEFS += -D_LINUX_SOURCE_COMPAT=1
+AM_CPPFLAGS += -D_LINUX_SOURCE_COMPAT=1
 endif
 
 # Set flags for NetBSD.