Use the Autoconf macro AC_USE_SYSTEM_EXTENSIONS

The Autoconf macro AC_USE_SYSTEM_EXTENSIONS defines preprocessor macros
which turn on extensions to C and POSIX. The macro also makes it easier
for developers to use the extensions without needing (or forgetting) to
define them manually.

The macro can be safely used because it was introduced in Autoconf 2.60
and Tor requires Autoconf 2.63 and above.