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__.