diff --git a/acinclude.m4.in b/acinclude.m4.in
index 29513f4..535e1d9 100644
--- a/acinclude.m4.in
+++ b/acinclude.m4.in
@@ -1233,7 +1233,7 @@ AC_DEFUN([KDE_USE_QT],
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_cxxflags_safe="$CXXFLAGS"
-  CXXFLAGS="-I/usr/include/tqt"
+  CXXFLAGS="$CXXFLAGS -I/usr/include/tqt"
   AC_TRY_COMPILE([
   #define TQT_VERSION_ONLY
   #include <tqt.h>