tests: enable parallel-tests support

* configure.ac (AM_INIT_AUTOMAKE): Add parallel-tests.
diff --git a/configure.ac b/configure.ac
index c4896f3..03e49fe 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6,7 +6,7 @@
 AC_CONFIG_SRCDIR([strace.c])
 AC_CONFIG_AUX_DIR([.])
 AC_CONFIG_HEADERS([config.h])
-AM_INIT_AUTOMAKE([foreign dist-xz no-dist-gzip silent-rules])
+AM_INIT_AUTOMAKE([foreign dist-xz no-dist-gzip silent-rules parallel-tests])
 AM_MAINTAINER_MODE
 AC_CANONICAL_HOST