bpo-39406: Implement os.putenv() with setenv() if available (GH-18128)

If setenv() C function is available, os.putenv() is now implemented
with setenv() instead of putenv(), so Python doesn't have to handle
the environment variable memory.

configure

@@ -782,7 +782,6 @@ infodir
 docdir
 oldincludedir
 includedir
-runstatedir
 localstatedir
 sharedstatedir
 sysconfdir
@@ -896,7 +895,6 @@ datadir='${datarootdir}'
 sysconfdir='${prefix}/etc'
 sharedstatedir='${prefix}/com'
 localstatedir='${prefix}/var'
-runstatedir='${localstatedir}/run'
 includedir='${prefix}/include'
 oldincludedir='/usr/include'
 docdir='${datarootdir}/doc/${PACKAGE_TARNAME}'
@@ -1149,15 +1147,6 @@ do
   | -silent | --silent | --silen | --sile | --sil)
     silent=yes ;;
 
-  -runstatedir | --runstatedir | --runstatedi | --runstated \
-  | --runstate | --runstat | --runsta | --runst | --runs \
-  | --run | --ru | --r)
-    ac_prev=runstatedir ;;
-  -runstatedir=* | --runstatedir=* | --runstatedi=* | --runstated=* \
-  | --runstate=* | --runstat=* | --runsta=* | --runst=* | --runs=* \
-  | --run=* | --ru=* | --r=*)
-    runstatedir=$ac_optarg ;;
-
   -sbindir | --sbindir | --sbindi | --sbind | --sbin | --sbi | --sb)
     ac_prev=sbindir ;;
   -sbindir=* | --sbindir=* | --sbindi=* | --sbind=* | --sbin=* \
@@ -1295,7 +1284,7 @@ fi
 for ac_var in	exec_prefix prefix bindir sbindir libexecdir datarootdir \
 		datadir sysconfdir sharedstatedir localstatedir includedir \
 		oldincludedir docdir infodir htmldir dvidir pdfdir psdir \
-		libdir localedir mandir runstatedir
+		libdir localedir mandir
 do
   eval ac_val=\$$ac_var
   # Remove trailing slashes.
@@ -1448,7 +1437,6 @@ Fine tuning of the installation directories:
   --sysconfdir=DIR        read-only single-machine data [PREFIX/etc]
   --sharedstatedir=DIR    modifiable architecture-independent data [PREFIX/com]
   --localstatedir=DIR     modifiable single-machine data [PREFIX/var]
-  --runstatedir=DIR       modifiable per-process data [LOCALSTATEDIR/run]
   --libdir=DIR            object code libraries [EPREFIX/lib]
   --includedir=DIR        C header files [PREFIX/include]
   --oldincludedir=DIR     C header files for non-gcc [/usr/include]
@@ -10303,6 +10291,7 @@ fi
 
 
 
+
 if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
 	if test -n "$ac_tool_prefix"; then
   # Extract the first word of "${ac_tool_prefix}pkg-config", so it can be a program name with args.
@@ -11561,7 +11550,7 @@ for ac_func in alarm accept4 setitimer getitimer bind_textdomain_codeset chown \
  posix_fallocate posix_fadvise posix_spawn posix_spawnp pread preadv preadv2 \
  pthread_condattr_setclock pthread_init pthread_kill putenv pwrite pwritev pwritev2 \
  readlink readlinkat readv realpath renameat \
- sem_open sem_timedwait sem_getvalue sem_unlink sendfile setegid seteuid \
+ sem_open sem_timedwait sem_getvalue sem_unlink sendfile setegid setenv seteuid \
  setgid sethostname \
  setlocale setregid setreuid setresuid setresgid setsid setpgid setpgrp setpriority setuid setvbuf \
  sched_get_priority_max sched_setaffinity sched_setscheduler sched_setparam \