Re: AC_INCLUDES_DEFAULT does not need to probe for headers that are part of C89 or POSIX.1 anymore

[Paul Eggert]

No doubt the set of default headers
should be modernized, but it's going too far, I expect,
to have Autoconf assumes that headers like <unistd.h>
exist.  Autoconf-generated scripts are sometimes used
to configure on MSVC platforms, for example, and MSVC
doesn't have <unistd.h>.