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

[Dane Springmeyer]

I've built build2 0.6.2 on OS X 10.12 and have encountered a hang during header extraction. The tool keeps running and never exits.

Profiling the hung "b" command indicates the hang is in build2::cc::compile::extract_headers.

The problem can be replicated with an include of  '#include </path/to/header.hpp>' instead of '#include "/path/to/header.hpp"' where the "/path/to/header.hpp" exists on the system.

I've posted a test case which I would expect to error (it simulates a developer mistake in the include syntax). But instead of producing an error, the test case hangs. It can be found at https://github.com/springmeyer/build2-testcase.

I built build2 with the `./build.sh` script and using the clang++ compiler:

$ clang++ -v
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.7.0

Hopefully this is enough to go on for the build2 developers to replicate and fix. If not, happy to provide more details.

Dane