tests: fix regressions.

Exit status 63 is documented for version-mismatch.
	* bootstrap.conf (gnulib_modules): Remove sysexits.
	* src/system.h (EX_MISMATCH): Define.
	* src/parse-gram.y (version_check): Use it instead of EX_CONFIG.

Missing includes.
	* tests/calc.at, tests/named-refs.at: Include assert.h.

tests/calc.at

@@ -207,6 +207,7 @@ extern FILE *input;]AT_SKEL_CC_IF([[
 
 %code
 {
+#include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 #if HAVE_UNISTD_H

tests/named-refs.at

@@ -23,6 +23,7 @@ AT_SETUP([Tutorial calculator])
 AT_DATA_GRAMMAR([test.y],
 [[
 %{
+#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>