This update is to make libsecp256k1 build on OpenBSD (more specifically OpenBSD 5.7 with Autotools 2.69).

Without the "AM_PROG_CC_C_O" line in configure.ac, ./autogen.sh crashes with "Makefile.am: C objects in subdir but AM_PROG_CC_C_O' not inconfigure.ac'\nautoreconf-2.69: automake failed with exit status: 1".

@theuni comments?

Further research shows that this was not an OpenBSD version problem (and not an Autotools version problem, as I understand it, also), but an Automake version problem: "AM_PROG_CC_C_O" is not required at first in Automake version 1.14, and that's a fairly recent version, and a lot of people may have older Automake versions.

In particular, it's extremely confusing to get this error (as in the build crashes because of the absence of "AM_PROG_CC_C_O") as generally which Automake version is required is not stated overly clearly in a project and you easily get a feel that the project is broken or at least not supported on your system.

libsecp256k1 is so much of an accessory library, that I'd say it'd be kind of odd to have particular Automake version requirements, so I'd suggest introducing the "AM_PROG_CC_C_O" line for Automake backward compatibility.

That macro is deprecated, but I don't see the harm in adding it if it fixes a build. Looks like worst-cast it's a nop.

Great - you can just write it as:

Legacy, for compatibility with Automake versions prior to 1.14

AM_PROG_CC_C_O