diff --git a/configure.in b/configure.in
index fb9e3c6..3b949e7 100644
--- a/configure.in
+++ b/configure.in
@@ -115,7 +115,7 @@ PKG_CHECK_MODULES([QT], [Qt5Core Qt5Gui Qt5Widgets Qt5Network],
     RCC=${RCC:-$(pkg-config --variable=rcc_location Qt5Core)}
     LUPDATE=${LUPDATE:-$(pkg-config --variable=lupdate_location Qt5Core)}
     LRELEASE=${LRELEASE:-$(pkg-config --variable=lrelease_location Qt5Core)}
-    CXXFLAGS+=" -DQT_NETWORK_LIB ${QT_CFLAGS}"
+    CPPFLAGS+=" -DQT_NETWORK_LIB ${QT_CFLAGS}"
     LIBS+=" ${QT_LIBS}"
     have_qt=1],
    [PKG_CHECK_MODULES([QT], [QtCore QtGui QtNetwork],
@@ -125,7 +125,7 @@ PKG_CHECK_MODULES([QT], [Qt5Core Qt5Gui Qt5Widgets Qt5Network],
         RCC=${RCC:-$(pkg-config --variable=rcc_location QtCore)}
         LUPDATE=${LUPDATE:-$(pkg-config --variable=lupdate_location QtCore)}
         LRELEASE=${LRELEASE:-$(pkg-config --variable=lrelease_location QtCore)}
-        CXXFLAGS+=" -DQT_NETWORK_LIB ${QT_CFLAGS}"
+        CPPFLAGS+=" -DQT_NETWORK_LIB ${QT_CFLAGS}"
         LIBS+=" ${QT_LIBS}"
         have_qt=1],
        [have_qt=0])])