DRAFT lib: Add putenv.

* lib/posix/putenv.c: New file.
* build-aux/configure-lib.sh (libc_gnu_SOURCES): Add it.
* include/stdlib.h (putenv): Add prototype.

build-aux/configure-lib.sh

@@ -331,6 +331,7 @@ lib/posix/alarm.c
 lib/posix/execl.c
 lib/posix/execlp.c
 lib/posix/mktemp.c
+lib/posix/putenv.c
 lib/posix/sbrk.c
 lib/posix/sleep.c
 lib/posix/unsetenv.c

include/stdlib.h

@@ -45,6 +45,7 @@ void _exit (int status);
 void exit (int status);
 void free (void *ptr);
 char *getenv (char const *s);
+int putenv (char *s);
 int setenv (char const *s, char const *v, int overwrite_p);
 int unsetenv (char const *name);
 void *malloc (size_t);

lib/posix/putenv.c

@@ -0,0 +1,35 @@
+/* -*-comment-start: "//";comment-end:""-*-
+ * GNU Mes --- Maxwell Equations of Software
+ * Copyright © 2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ *
+ * This file is part of GNU Mes.
+ *
+ * GNU Mes is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * GNU Mes is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <mes/lib.h>
+#include <string.h>
+#include <stdlib.h>
+
+int
+putenv (char *s)
+{
+  char *value = strchr (s, '=');
+  if (!value)
+    return unsetenv (s);
+
+  *value++ = '0';
+  value = "";
+  return setenv (s, value, 1);
+}