I get the following error when building open-vm-tools{,-nox11}:

================
utilBacktrace.c:156:20: error: implicit declaration of function '_Unwind_GetCFA' is invalid in C99
      [-Werror,-Wimplicit-function-declaration]
   uintptr_t cfa = _Unwind_GetCFA(ctx);
                   ^
utilBacktrace.c:358:4: error: implicit declaration of function '_Unwind_Backtrace' is invalid in C99
      [-Werror,-Wimplicit-function-declaration]
   _Unwind_Backtrace(UtilBacktraceFromPointerCallback, &data);
================

This happens if the build detects libunwind (and adds -DHAVE_LIBUNWIND_H=1 during build).  Reproduce by installing the libunwind port and trying to build open-vm-tools*.

If unwind.h is not available, you don't get the error.

Or if the libunwind port is installed, and you add -D_GNU_SOURCE, then you don't get the error.

Or if the unwind.h is detected and you build with gcc5 or gcc6, then you don't get the error.  Those gcc ports both have an internal version of unwind.h

See also https://lists.freebsd.org/pipermail/freebsd-virtualization/2017-January/005186.html

I don't think it's vital for a working open-vm-tools to have unwind enabled.  So disabling unwind at configure time seems right until all supported compilers support unwind.  There's no configure switch for --without-unwind or anything, so it'd be a patch against configure.

Alternately, we could pass in -D_GNU_SOURCE.

I can put together a patch.  Let me know what you think about the above and which option seems best.