Avoid entering static libusb directory if --with-system-libusb is enabled.

compat/Makefile.am

@@ -2,5 +2,7 @@
 SUBDIRS	= jansson-2.5
 
 if WANT_USBUTILS
+if WANT_STATIC_LIBUSB
 SUBDIRS += libusb-1.0
 endif
+endif

configure.ac

@@ -368,6 +368,8 @@ else
 	LIBUSB_LIBS=""
 fi
 
+AM_CONDITIONAL([WANT_STATIC_LIBUSB], [test x$dlibusb = xno])
+
 AC_CONFIG_SUBDIRS([compat/jansson-2.5])
 JANSSON_LIBS="compat/jansson-2.5/src/.libs/libjansson.a"