I noticed this when doing some M4 macro analysis work, and saw
that the version of `ax_check_compile_flag.m4` in Tor has two serial
lines ('serial 5' and 'serial 6') which is invalid.

We could just fix one of the lines, but it makes more sense to just
sync with upstream, I think.

Import ax_check_compile_flag.m4 from autoconf-archive at latest
commit at time of writing (4e8aab846b0872fba99f1fe02ebcdff178a34c87).

Signed-off-by: Sam James <sam@gentoo.org>

update AX_CHECK_COMPILE_FLAG function call

This changes the behaviour of `tor --version` in such a way.
```console
src/app/tor --version
Tor version 0.4.5.1-alpha-dev (git-46ccde66).
Tor is running on Linux with Libevent 2.1.12-stable, OpenSSL 1.1.1h, Zlib 1.2.11, Liblzma 5.2.4, Libzstd 1.4.5 and Glibc 2.31 as libc.
Tor compiled with GCC version 10.2.0
```

Fixes #32102

Samanta Navarro authored Nov 11, 2020 and David Goulet committed Nov 12, 2020

Typos found with codespell.

Please keep in mind that this should have impact on actual code
and must be carefully evaluated:

src/core/or/lttng_circuit.inc
-    ctf_enum_value("CONTROLER", CIRCUIT_PURPOSE_CONTROLLER)
+    ctf_enum_value("CONTROLLER", CIRCUIT_PURPOSE_CONTROLLER)

David Goulet authored Aug 08, 2019 and Nick Mathewson committed Oct 17, 2019

Signed-off-by: David Goulet <dgoulet@torproject.org>

cypherpunks authored Jun 17, 2016 and Nick Mathewson committed Jun 17, 2016

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.

This beats our old implementation, which wouldn't work when cross-compiling