Bugfix: configure: if blocks require at least one command, so fill with true

configure.ac

@@ -193,6 +193,7 @@ PKG_CHECK_MODULES([USB], [libusb-1.0],[
 	done
 	if test "x$libusb" = xyes; then
 			AC_CHECK_DECL([libusb_init],[
+				true
 			],[
 				AC_MSG_CHECKING([whether libusb_init is declared in subdirectory])
 				libusb_include_path=`echo '#include <libusb-1.0/libusb.h>' | cpp -M 2>/dev/null | sed 's/^[^[:space:]]\+[[:space:]]\+\([^[:space:]]\+\)\/.*$/\1/;t;d'`