diff --git a/cmd/zdb/Makefile.am b/cmd/zdb/Makefile.am
index fd12fb7dbe..8645837df5 100644
--- a/cmd/zdb/Makefile.am
+++ b/cmd/zdb/Makefile.am
@@ -1,6 +1,7 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES  = -I${top_srcdir}/lib/libspl/include
+DEFAULT_INCLUDES += \
+	-I${top_srcdir}/lib/libspl/include
 
 sbin_PROGRAMS = zdb
 
diff --git a/cmd/zdump/Makefile.am b/cmd/zdump/Makefile.am
index 73d718f736..87a7dd495b 100644
--- a/cmd/zdump/Makefile.am
+++ b/cmd/zdump/Makefile.am
@@ -1,6 +1,7 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES  = -I${top_srcdir}/lib/libspl/include
+DEFAULT_INCLUDES += \
+	-I${top_srcdir}/lib/libspl/include
 
 sbin_PROGRAMS = zdump
 
diff --git a/cmd/zfs/Makefile.am b/cmd/zfs/Makefile.am
index 871f058855..33258daa68 100644
--- a/cmd/zfs/Makefile.am
+++ b/cmd/zfs/Makefile.am
@@ -1,6 +1,7 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES  = -I${top_srcdir}/lib/libspl/include
+DEFAULT_INCLUDES += \
+	-I${top_srcdir}/lib/libspl/include
 
 sbin_PROGRAMS = zfs
 
diff --git a/cmd/zinject/Makefile.am b/cmd/zinject/Makefile.am
index 632973d147..36185bfae7 100644
--- a/cmd/zinject/Makefile.am
+++ b/cmd/zinject/Makefile.am
@@ -1,6 +1,7 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES  = -I${top_srcdir}/lib/libspl/include
+DEFAULT_INCLUDES += \
+	-I${top_srcdir}/lib/libspl/include
 
 sbin_PROGRAMS = zinject
 
diff --git a/cmd/zpool/Makefile.am b/cmd/zpool/Makefile.am
index 81911828c4..97af752327 100644
--- a/cmd/zpool/Makefile.am
+++ b/cmd/zpool/Makefile.am
@@ -1,6 +1,7 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES  = -I${top_srcdir}/lib/libspl/include
+DEFAULT_INCLUDES += \
+	-I${top_srcdir}/lib/libspl/include
 
 sbin_PROGRAMS = zpool
 
diff --git a/cmd/ztest/Makefile.am b/cmd/ztest/Makefile.am
index 6df8699532..14f9dc4315 100644
--- a/cmd/ztest/Makefile.am
+++ b/cmd/ztest/Makefile.am
@@ -1,6 +1,7 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES  = -I${top_srcdir}/lib/libspl/include
+DEFAULT_INCLUDES += \
+	-I${top_srcdir}/lib/libspl/include
 
 sbin_PROGRAMS = ztest
 
diff --git a/config/Rules.am b/config/Rules.am
index 0c5e9f203f..92815e9033 100644
--- a/config/Rules.am
+++ b/config/Rules.am
@@ -1,3 +1,5 @@
+DEFAULT_INCLUDES = -I${top_srcdir}
+
 # FIXME: Quiet warnings not covered by the gcc-* patches.  We should
 # FIXME: consider removing this as soon as we reasonably can.
 AM_CFLAGS  = -Wall -Wstrict-prototypes -Wshadow -Werror
diff --git a/lib/libavl/Makefile.am b/lib/libavl/Makefile.am
index 6b8f0f6f75..3ae2cb98a7 100644
--- a/lib/libavl/Makefile.am
+++ b/lib/libavl/Makefile.am
@@ -1,6 +1,6 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES = \
+DEFAULT_INCLUDES += \
 	-I${top_srcdir}/module/avl/include \
 	-I${top_srcdir}/lib/libspl/include
 
diff --git a/lib/libnvpair/Makefile.am b/lib/libnvpair/Makefile.am
index c19a169bfa..28471bf5df 100644
--- a/lib/libnvpair/Makefile.am
+++ b/lib/libnvpair/Makefile.am
@@ -1,6 +1,6 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES = \
+DEFAULT_INCLUDES += \
 	-I${top_srcdir}/module/nvpair/include \
 	-I${top_srcdir}/lib/libnvpair/include \
 	-I${top_srcdir}/lib/libspl/include
diff --git a/lib/libuutil/Makefile.am b/lib/libuutil/Makefile.am
index 6b20eca678..78eb75e27f 100644
--- a/lib/libuutil/Makefile.am
+++ b/lib/libuutil/Makefile.am
@@ -1,6 +1,6 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES = \
+DEFAULT_INCLUDES += \
 	-I${top_srcdir}/lib/libuutil/include \
 	-I${top_srcdir}/lib/libspl/include \
 	-I${top_srcdir}/module/avl/include
diff --git a/lib/libzfs/Makefile.am b/lib/libzfs/Makefile.am
index 4996c48e1d..d4da81dcfd 100644
--- a/lib/libzfs/Makefile.am
+++ b/lib/libzfs/Makefile.am
@@ -1,6 +1,6 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES = \
+DEFAULT_INCLUDES += \
 	-I${top_srcdir}/lib/libzfs/include \
 	-I${top_srcdir}/lib/libzpool/include \
 	-I${top_srcdir}/lib/libspl/include \
diff --git a/lib/libzpool/Makefile.am b/lib/libzpool/Makefile.am
index 2a6d49af3c..f3f69e1a54 100644
--- a/lib/libzpool/Makefile.am
+++ b/lib/libzpool/Makefile.am
@@ -1,6 +1,6 @@
 include $(top_srcdir)/config/Rules.am
 
-DEFAULT_INCLUDES = \
+DEFAULT_INCLUDES += \
         -I${top_srcdir}/lib/libzpool/include \
         -I${top_srcdir}/lib/libspl/include \
         -I${top_srcdir}/lib/libnvpair/include \