[2.7] bpo-32616: Disable computed gotos by default for clang < 5

[methane]

https://bugs.python.org/issue32616

[benjaminp]

We should probably put this check in configure.ac, so that configure doesn't claim we're going to use computed use gotos while we silently don't. (You could just stick the ifdef into the AC_RUN_IFELSE.)

[methane]

I'm sorry, I'm not familiar with configure.ac.
As my understanding, configure just define HAVE_COMPUTED_GOTOS automatically.
USE_COMPUTED_GOTOS is never defined automatically, unless --with(out)-computed-gotos
is specified.

This pull request is just change behavior when USE_COMPUTED_GOTOS is not defined.

[methane]

This is output of configure script:

checking for socklen_t... yes
checking for build directories... checking for --with-computed-gotos... no value specified
checking whether gcc -pthread supports computed gotos... yes
done

[ilovezfs]

@methane @benjaminp unfortunately this issue still affects Python 2.7.15rc1 because in Xcode 9.x, the value of __clang_major__ is 9.

[methane]

@ilovezfs We know, but there is no easy fix for it. See https://bugs.python.org/issue32616 for continued discussion.

[p0six]

So is this now considered resolved for High Sierra users via 2.7.15rc1?

[ilovezfs]

@p0six no. You still must manually pass --without-computed-gotos to configure if you're building from source.

[ilovezfs]

@p0six however, Xcode >= 9.3 is apparently unaffected.