diff --git a/configure.in b/configure.in
index 3b949e7..935e2e4 100644
--- a/configure.in
+++ b/configure.in
@@ -99,7 +99,7 @@ case $host in
     ;;
   (*mingw*|*win*)
     LIBS+=" -lwinhttp"
-    CXXFLAGS+=" -DQ_OS_WIN32"
+    CPPFLAGS+=" -DQ_OS_WIN32"
     ;;
   (*)
     PKG_CHECK_MODULES([PROXY], [libproxy-1.0],