maint: avoid "magic number exit".

* cfg.mk (local-checks-to-skip): No longer skip it.
	* bootstrap.conf (gnulib_modules): Add sysexits.
	* doc/bison.texinfo, etc/bench.pl.in, src/parse-gram.y,
	* src/system.h, tests/calc.at, tests/named-refs.at: Use assert
	where appropriate instead of "if (...) exit".
	Use symbolic exit status elsewhere.

src/parse-gram.y

@@ -804,7 +804,7 @@ version_check (location const *loc, char const *version)
     {
       complain_at (*loc, "require bison %s, but have %s",
                    version, PACKAGE_VERSION);
-      exit (63);
+      exit (EX_CONFIG);
     }
 }
 

src/system.h

@@ -52,6 +52,7 @@
 typedef size_t uintptr_t;
 #endif
 
+#include <sysexits.h>
 
 /*---------.
 | Gnulib.  |