build: Improve --with-system-libc support.

* build-aux/build-lib.sh: Avoid building libc for --with-system-libc.
Make libmes richer.
build-aux/check-mescc.sh: Enable 50-malloc test.
* include/dirstream.h: Use HAVE_DIRSTREAM_H to cater for non-existing
dirstream.h.
* include/grp.h[SYSTEM_LIBC]: Typo.
* include/mes/lib.h (__brk, __call_at_exit)[SYSTEM_LIBC]: Declare.
* include/stdlib.h (comparison_fn_t)[SYSTEM_LIBC]: Declare.
* include/time.h (mktime): Return time_t.
* lib/posix/isatty.c [SYSTEM_LIBC]: Do not (re)declare termios.
* lib/stub/mktime.c (mktime): Return time_t.
* lib/tests/scaffold/41-ternary.c (main): Remove assumption that
locals are initialized.
* lib/tests/scaffold/70-stdarg.c: Cater for SYSTEM_LIBC.
* lib/tests/setjmp/80-setjmp.c: Declare jmp buf.
* lib/*/*.c: Add missing mes/lib.h, errno.h includes.

include/time.h

@@ -59,7 +59,7 @@ struct timespec
 int clock_gettime (clockid_t clk_id, struct timespec *tp);
 struct tm *localtime (time_t const *timep);
 struct tm *gmtime (time_t const *time);
-int mktime (struct tm *broken_time);
+time_t mktime (struct tm *broken_time);
 int nanosleep (struct timespec const *requested_time, struct timespec const *remaining);
 time_t time (time_t * tloc);
 

lib/stub/getgrgid.c

@@ -19,6 +19,7 @@
  */
 
 #include <mes/lib.h>
+#include <errno.h>
 #include <grp.h>
 
 struct group *
@@ -27,8 +28,13 @@ getgrgid (gid_t gid)
   static int stub = 0;
   if (__mes_debug () && !stub)
     eputs ("getgrid stub\n");
-  static char *groups[2] = {"root", 0 };
+  static char *groups[2] = { "root", 0 };
-  static struct group root = {"root", 0, groups};
+#if SYSTEM_LIBC
+  static struct group root = { "root", 0, 0 };
+  root.gr_mem = groups;
+#else
+  static struct group root = { "root", 0, groups };
+#endif
   stub = 1;
   errno = 0;
   return &root;

lib/stub/getgrnam.c

@@ -19,6 +19,7 @@
  */
 
 #include <mes/lib.h>
+#include <errno.h>
 #include <grp.h>
 
 struct group *
@@ -27,8 +28,13 @@ getgrnam (char const *name)
   static int stub = 0;
   if (__mes_debug () && !stub)
     eputs ("getgrid stub\n");
-  static char *groups[2] = {"root", 0 };
+  static char *groups[2] = { "root", 0 };
-  static struct group root = {"root", 0, groups};
+#if SYSTEM_LIBC
+  static struct group root = { "root", 0, 0 };
+  root.gr_mem = groups;
+#else
+  static struct group root = { "root", 0, groups };
+#endif
   stub = 1;
   errno = 0;
   return &root;

lib/stub/getpgid.c

@@ -19,6 +19,7 @@
  */
 
 #include <mes/lib.h>
+#include <errno.h>
 #include <unistd.h>
 
 int

lib/stub/getpgrp.c

@@ -19,6 +19,7 @@
  */
 
 #include <mes/lib.h>
+#include <errno.h>
 #include <unistd.h>
 
 pid_t

lib/stub/mktime.c

@@ -19,9 +19,10 @@
  */
 
 #include <mes/lib.h>
+#include <errno.h>
 #include <time.h>
 
-int
+time_t
 mktime (struct tm *broken_time)
 {
   static int stub = 0;

lib/stub/setgrent.c

@@ -19,6 +19,7 @@
  */
 
 #include <mes/lib.h>
+#include <errno.h>
 
 void
 setgrent (void)