Add CI using GitHub Actions

[bonachea]

Deploy some basic CI in Actions to serve as a (relatively lightweight) smoke test

[PHHargrove]

LGTM.

Curious if --disable-ccs-rpc for Xcode 26 is present for coverage of that case, or if it is necessary on that target as the current comment implies. PR is APPROVED regardless, though an explanatory comment would be appreciated.

[bonachea]

@PHHargrove asked:

Curious if --disable-ccs-rpc for Xcode 26 is present for coverage of that case, or if it is necessary on that target as the current comment implies. PR is APPROVED regardless, though an explanatory comment would be appreciated.

  # XCode 26 / gcc bug workaround: 
  # <mach/mach_time.h> and <mach/mach.h> generate xnu_static_assert_struct_size errors from <mach/message.h>
  if [[ ${{ matrix.os }} =~ macos-26 ]] ; then
    echo "CONFIGURE_ARGS=$CONFIGURE_ARGS --enable-force-posix-realtime --disable-ccs-rpc" >> "$GITHUB_ENV"
  fi

Both configure options are workarounds for the same underlying problem, namely the brew installs of GNU G++ are incapable of compiling the <mach/message.h> header installed on GitHub's macos-26-* runners. Timers reach the failure on the problematic header via include of <mach/mach_time.h> and CCS reaches it via include of <mach/mach.h>.

I feel like the existing comment already conveys this information, but if you disagree then please use the suggest feature to specify your concrete suggested improvement.

The runners were upgraded yesterday with a new version of XCode 26, but I've just re-confirmed here that the problem still exists when we don't disable CCS (and AMUDP fails in a similar way when Mach timers are enabled).