extensions: port better to MINUX 3, HP-UX, autoheader 2.62

extensions: port better to MINUX 3, HP-UX, autoheader 2.62

Some of these changes are merged in from git Autoconf.
* m4/extensions.m4 (AC_USE_SYSTEM_EXTENSIONS):
When deciding whether to define _XOPEN_SOURCE, inspect the
preprocessor macro __hpux instead of the more-heavyweight
operation of requiring AC_CANONICAL_HOST.  Define _NETBSD_HOST on
MINUX, for MINUX 3.  Use USE_SYSTEM_EXTENSIONS, not __EXTENSIONS__,
as the key for __EXTENSIONS__.