maint: stdbuf.c: avoid the OS X putenv function

maint: stdbuf.c: avoid the OS X putenv function

* bootstrap.conf (gnulib_modules): Add setenv, to make this
module dependency explicit; setenv is also used by split.
* src/stdbuf.c (set_LD_PRELOAD) [__APPLE__]: Use the OS X setenv
function, rather than putenv, per that documentation:
https://developer.apple.com/\
library/mac/documentation/Darwin/Reference/ManPages/man3/putenv.3.html