diff --git a/autoconf/variables.mak.in b/autoconf/variables.mak.in
index b67e467..6022434 100644
--- a/autoconf/variables.mak.in
+++ b/autoconf/variables.mak.in
@@ -65,7 +65,7 @@ DRVLIBS = @PTHREAD_LFLAGS@ @DRVLIBS@
 X_LIBS = @X_LIBS@
 X_EXTRA_LIBS = @X_EXTRA_LIBS@
 
-CPPFLAGS = @CPPFLAGS@ -I$(topdir)/include $(EXTRAINCS)
+CPPFLAGS = -I$(topdir)/include @CPPFLAGS@ $(EXTRAINCS)
 CFLAGS = $(CPPFLAGS) @CFLAGS@ @PTHREAD_CFLAGS@
 CXXFLAGS = $(CPPFLAGS) @CXXFLAGS@ @PTHREAD_CFLAGS@
 LDFLAGS = @LDFLAGS@