Re: doc putenv portability

[Akim Demaille]

 >         * autoconf.texi (Function Portability): Add putenv and unsetenv.
 > `putenv'
 >      POSIX specifies that `putenv' puts the given string directly in
 >      `environ', but some systems make a copy of it instead (eg.  glibc
 >      2.0, or BSD).  And when a copy is made, `unsetenv' might not free
 >      it, causing a memory leak (eg. FreeBSD 4).

 >      POSIX specifies that `putenv("FOO")' removes `FOO' from the
 >      environment, but on some systems (eg. FreeBSD 4) this is not the
 >      case and instead `unsetenv' must be used.

 >      On MINGW, a call `putenv("FOO=")' removes `FOO' from the
 >      environment, rather than inserting it with an empty value.

 > `unsetenv'
 >      On MINGW, `unsetenv' is not available, but a variable `FOO' can be
 >      removed with a call `putenv("FOO=")', as described under `putenv'
 >      above.

Please, install.  Thanks!