Index: configure.ac
===================================================================
--- configure.ac
+++ configure.ac
@@ -179,6 +179,10 @@
 dnl
 dnl Checks for Qt libraries.  Deal correctly with $pinentry_qt = maybe.
 dnl
+dnl The Qt macros call AC_LANG_PUSH(C++) which in turn calls AC_PROG_CXX,
+dnl which unfortunately cannot be called conditionally -> let's do it here
+dnl no matter if it's going to be used or not :(
+AC_PROG_CXX
 if test "$pinentry_qt" != "no"; then
 QT_CHECK_COMPILERS
 QT_PATH