bitcoin_qt.m4: Use correct M4 quoting characters.

build-aux/m4/bitcoin_qt.m4

@@ -254,10 +254,10 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
       bitcoin_enable_qt_dbus=yes
     fi
     if test x$use_dbus = xyes && test x$have_qt_dbus = xno; then
-      AC_MSG_ERROR("libQtDBus not found. Install libQtDBus or remove --with-qtdbus.")
+      AC_MSG_ERROR([libQtDBus not found. Install libQtDBus or remove --with-qtdbus.])
     fi
     if test x$LUPDATE = x; then
-      AC_MSG_WARN("lupdate is required to update qt translations")
+      AC_MSG_WARN([lupdate is required to update qt translations])
     fi
   ],[
     bitcoin_enable_qt=no