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.
This M4 module lets us learn the right way (out of at least 18
possibilities) to extract the current PC for stack-trace-fixup-in-signal
purposes. The Google Performance Tools license is 3-clause BSD.
