diff --git a/configure.ac b/configure.ac
index 9841864..e835083 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,7 +20,7 @@ AX_BUILD_EXAMPLES
 # qt features
 AX_CHECK_QT([QTGUI], [QtCore QtGui], [QtWidgets])
 AX_CHECK_QT([QTNETWORK], [QtNetwork])
-AM_CPPFLAGS+=" -DQT_NO_KEYWORDS"
+CPPFLAGS="${CPPFLAGS} -DQT_NO_KEYWORDS"
 
 # libraries
 AC_ARG_ENABLE(pkcs11-download,