sane-find-scanner: disable USB if not supported

This fixes a [-Wpedantic] compiler warning about ISO C forbidding empty
translation units.  The check-usb-chip.c file reduces to such a file in
the case USB support was explicitly disabled or no usable library found.

configure.ac

@@ -426,7 +426,7 @@ AS_IF([test xno != "x$with_usb"],
 AS_IF([test xyes = "x$with_usb" && test xyes != "x$have_usb"],
   [AC_MSG_ERROR([USB support requested but required libraries not found.])
   ])
-dnl AM_CONDITIONAL([have_usb], [test x != "x$USB_LIBS])
+AM_CONDITIONAL([have_usblib], [test x != "x$USB_LIBS"])
 
 dnl ************
 dnl SCSI Support

tools/Makefile.am

@@ -34,7 +34,10 @@ CLEANFILES = $(bin_SCRIPTS) $(dist_noinst_SCRIPTS)
 EXTRA_DIST = check-po.awk libtool-get-dll-ext mustek600iin-off.c \
 	     RenSaneDlls.cmd README xerox
 
-sane_find_scanner_SOURCES = sane-find-scanner.c check-usb-chip.c
+sane_find_scanner_SOURCES = sane-find-scanner.c
+if have_usblib
+sane_find_scanner_SOURCES += check-usb-chip.c
+endif
 sane_find_scanner_LDADD = ../sanei/libsanei.la ../lib/liblib.la \
 			  $(USB_LIBS) $(IEEE1284_LIBS) $(SCSI_LIBS) \
 			  ../backend/sane_strstatus.lo