diff --git a/Makefile.vc b/Makefile.vc
index 1c2ab7df..c9357ba2 100644
--- a/Makefile.vc
+++ b/Makefile.vc
@@ -45,6 +45,7 @@ OUTDIR = $(OBJDIR)
 !ENDIF
 
 !IF "$(HAVE_AVX2)" == "1"
+CFLAGS += /DWEBP_HAVE_AVX2
 AVX2_FLAGS = /arch:AVX2
 !ENDIF
 
diff --git a/makefile.unix b/makefile.unix
index 28462954..5e2ed162 100644
--- a/makefile.unix
+++ b/makefile.unix
@@ -71,6 +71,7 @@ EXTRA_FLAGS += -Wshadow
 
 # AVX2-specific flags:
 ifeq ($(HAVE_AVX2), 1)
+EXTRA_FLAGS += -DWEBP_HAVE_AVX2
 src/dsp/%_avx2.o: EXTRA_FLAGS += -mavx2
 endif