configure: Allow building bench_bitcoin by itself

configure.ac

@@ -1003,8 +1003,8 @@ else
   AC_MSG_RESULT([no])
 fi
 
-if test x$build_bitcoin_utils$build_bitcoin_libs$build_bitcoind$bitcoin_enable_qt$use_tests = xnonononono; then
+if test x$build_bitcoin_utils$build_bitcoin_libs$build_bitcoind$bitcoin_enable_qt$use_bench$use_tests = xnononononono; then
-  AC_MSG_ERROR([No targets! Please specify at least one of: --with-utils --with-libs --with-daemon --with-gui or --enable-tests])
+  AC_MSG_ERROR([No targets! Please specify at least one of: --with-utils --with-libs --with-daemon --with-gui --enable-bench or --enable-tests])
 fi
 
 AM_CONDITIONAL([TARGET_DARWIN], [test x$TARGET_OS = xdarwin])