Missing includes for macOS build

I attempted to build this library on macOS (Xcode 7.3.1) and found that there were a few includes missing:

• example/assert_handler.cpp: missing <cstdlib> for std::exit()
• example/terminate_handler.cpp: missing <cstdlib> for std::exit()
• include/boost/stacktrace/detail/backend_linux.hpp: missing <signal.h> for kill()

Aside from that the code seems to work great on macOS (tested on 10.11.6 with Xcode 7.3.1 -- clang-703.0.31) using the Linux backend, based on my limited testing using your examples and test code.

Thanks!

[apolukhin]

Fixed.

Great thanks for the report and hints on how to fix the issue.