m4/nut_check_libmodbus.m4: make it visible when some libmodbus is found, but does not support libusb while we want it (and will fail NUT configure later) [networkupstools#2063]

Signed-off-by: Jim Klimov <[email protected]>

Author: jimklimov

m4/nut_check_libmodbus.m4

@@ -72,7 +72,12 @@ if test -z "${nut_have_libmodbus_seen}"; then
 	AC_CHECK_FUNCS(modbus_set_byte_timeout, [], [nut_have_libmodbus=no])
 	AC_CHECK_FUNCS(modbus_set_response_timeout, [], [nut_have_libmodbus=no])
 
-	AC_CHECK_FUNCS(modbus_new_rtu_usb, [nut_have_libmodbus_usb=yes], [nut_have_libmodbus_usb=no])
+	AC_CHECK_FUNCS(modbus_new_rtu_usb, [nut_have_libmodbus_usb=yes], [
+		AS_IF([test x"${nut_with_usb}" = xyes && test x"${nut_with_modbus}" = xyes && test x"${nut_have_libmodbus}" = xyes ], [
+			AC_MSG_WARN([Both --with-modbus and --with-usb were requested, and a libmodbus was found, but it seems to not support USB. You may require a custom build per https://github.com/networkupstools/nut/wiki/APC-UPS-with-Modbus-protocol])
+		])
+		nut_have_libmodbus_usb=no
+	])
 
 	dnl modbus_set_byte_timeout() and modbus_set_response_timeout()
 	dnl in 3.0.x and 3.1.x have different args (since ~2013): the