[build2] hang in build2::cc::compile::extract_headers

[Boris Kolpackov]

Dane Springmeyer <dane at mapbox.com> writes:

> I've built build2 0.6.2 on OS X 10.12 and have encountered a hang
> during header extraction.

I've managed to reproduce this, thanks for the test case! We've
actually hit a similar situation when running out of disk space
so in the latest staged version it no longer hangs but issues
this diagnostics:

error: inconsistent C++ compiler behavior
  info: clang++ -w -MQ ^ -x c++ -MD -E -frewrite-includes -MF - -o /tmp/build2-testcase/test/unit.o.ii /tmp/build2-testcase/test/unit.cxx
  info: clang++ -w -MQ ^ -x c++ -M -MG /tmp/build2-testcase/test/unit.cxx
  info: perhaps you are running out of disk space?
  info: while extracting header dependencies from test/cxx{unit}
  info: while applying rule cxx.compile to update test/obje{unit}
  info: while applying rule cxx.link to update test/exe{run-test}
  info: while applying rule alias to update dir{test/}
  info: while applying rule alias to update dir{./}

Digging further revealed that this is a bug in Clang which I've
reported:

https://bugs.llvm.org/show_bug.cgi?id=35580

If you would like to try the latest stage, you can get it here:

https://stage.build2.org/0/

Thanks,
Boris