I'm running an appveyor script that tests libbson and libmongoc on various platforms.
Everything works, including libmongoc API, except `bson_iter_binary`.

The log below is not very helpful, since by the time `bson_iter_binary` is called, the thread just panics and no output or error trace is printed.

In this case, libbson and libmongoc were built with appveyor's Visual Studio 2015 image.

```
Build started
git clone -q --branch=fn/windows https://github.com/felipenoris/Mongoc.jl.git C:\projects\mongoc-jl
git checkout -qf 176c69b415380ef89aad9dfdcc5f770dcec28b9f
Running Install scripts
iex ((new-object net.webclient).DownloadString("https://raw.githubusercontent.com/JuliaCI/Appveyor.jl/version-1/bin/install.ps1"))
Installing Julia...
Julia Version 1.0.3
Commit 099e826241 (2018-12-18 01:34 UTC)
Platform Info:
OS: Windows (x86_64-w64-mingw32)
CPU: Intel(R) Xeon(R) CPU E5-2697 v3 @ 2.60GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-6.0.0 (ORCJIT, haswell)
Environment:
JULIA_PROJECT = @.
Starting MongoDB
echo "%JL_BUILD_SCRIPT%"
"using Pkg; Pkg.build()"
C:\julia\bin\julia -e "%JL_BUILD_SCRIPT%"
Cloning default registries into C:\Users\appveyor\.julia\registries
Cloning registry General from "https://github.com/JuliaRegistries/General.git"
Installed BinaryProvider ─ v0.5.3
Building Mongoc → `C:\projects\mongoc-jl\deps\build.log`
Resolving package versions...
echo "%JL_TEST_SCRIPT%"
"using Pkg; Pkg.test(coverage=true)"
C:\julia\bin\julia -e "%JL_TEST_SCRIPT%"
Testing Mongoc
Resolving package versions...
┌ Info: Testing for platform:
└ platform_key_abi() = Windows(:x86_64, compiler_abi=CompilerABI(:gcc4, :cxx11))
┌ Warning: Failed to remove test file C:\projects\mongoc-jl\test\data.bson: unlink: resource busy or locked (EBUSY)
└ @ Main C:\projects\mongoc-jl\test\bson_tests.jl:327
Test Summary: | Pass Total
BSON | 10097 10097
Server Mongo Version: 4.0.1
ERROR: Package Mongoc errored during testing
Stacktrace:
[1] pkgerror(::String, ::Vararg

{String,N}

where N) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\Types.jl:120
[2] #test#65(::Bool, ::Function, ::Pkg.Types.Context, ::Array

{Pkg.Types.PackageSpec,1}) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\Operations.jl:1357
[3] #test at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:0 [inlined]
[4] #test#42(::Bool, ::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::Function, ::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}

) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:273
[5] #test#41 at .\none:0 [inlined]
[6] #test at .\none:0 [inlined]
[7] #test#38 at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\Pkg\src\API.jl:252 [inlined]
[8] (::getfield(Pkg.API, Symbol("#kw##test")))(::NamedTuple{(:coverage,),Tuple{Bool}}, ::typeof(Pkg.API.test)) at .\none:0
[9] top-level scope at none:0
```

Ref:

• builder: https://github.com/felipenoris/mongo-c-driver-builder
• client: https://github.com/felipenoris/Mongoc.jl