Compilation fails in Sys.cpp on mingw/windows

On Win 10, after installing the haxelib `minimingw` I'm setting the environment variable `toolchain=mingw` and try to compile a project with:

```
> haxe -cpp bin/cpp -D HXCPP_VERBOSE -lib travix -lib dataclass tests.hxml
```

tests.hxml:

```
-lib buddy
-lib deepstate
-cp ./tests/
-cp ./src/
-main Tests2
```

But it eventually fails with:

```
haxelib run hxcpp Build.xml haxe -DHXCPP_VERBOSE="1" -Dasynctools="0.1.0" -Dbuddy="2.10.0" -Dbuddy-colors="1" -Dbuddy_colors="1" -Ddataclass="1.5.1" -Ddataclass-date-auto-conversion="1" -Ddataclass_date_auto_conversion="1" -Ddeepstate="0.13.2" -Dhaxe3="1" -Dhaxe4="1" -Dhaxe_ver="4.000" -Dhxcpp_api_level="400" -Dpromhx="1.1.0" -Dpure="1" -Dsource-header="Generated by Haxe 4.0.0-rc.2+77068e10c" -Dstatic="1" -Dtarget.name="cpp" -Dtarget.static="true" -Dtarget.sys="true" -Dtarget.threaded="true" -Dtarget.utf16="true" -Dtink_chunk="0.2.0" -Dtink_cli="0.4.1" -Dtink_core="1.21.0" -Dtink_io="0.6.2" -Dtink_macro="0.17.4" -Dtink_streams="0.3.2" -Dtink_stringly="0.3.1" -Dtravix="0.12.2" -Duse-rtti-doc="1" -Duse_rtti_doc="1" -Dutf16="1" -I"./src/" -I"./tests/" -I"E:\\Projects\\Haxe\\deepstate\\src\\" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\promhx/1,1,0/src/main/" -I"E:\\Projects\\Haxe\\asynctools\\src\\" -I"e:\\Projects\\Haxe\\buddy\\src\\" -I"e:\\Projects\\Haxe\\dataclass\\src\\" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\tink_stringly/0,3,1/src/" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\tink_macro/0,17,4/src/" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\tink_core/1,21,0/src/" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\tink_streams/0,3,2/src/" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\tink_chunk/0,2,0/src/" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\tink_io/0,6,2/src/" -I"e:\\Program\\HaxeToolkit\\haxe\\lib\\tink_cli/0,4,1/src/" -I"E:\\Projects\\Haxe\\travix\\src\\" -I"E:\\Program\\HaxeToolkit\\haxe\\extraLibs/" -I"" -I"E:\\Program\\HaxeToolkit\\haxe\\std/cpp/_std/" -I"E:\\Program\\HaxeToolkit\\haxe\\std/"
__
/\ \
\ \ \___ __ _ ___ _____ _____
\ \ _ `\ /\ \/'\ /'___\/\ '__`\/\ '__`\
\ \ \ \ \\/> _hx_std_sys_env()':
e:/Program/HaxeToolkit/haxe/lib/hxcpp/4,0,8/src/hx/libs/std/Sys.cpp:722:15: error: 'environ' was not declared in this scope
char **e = environ;
^
Error: Build failed
```

Since the errors happen in Sys.cpp, I'm at a loss here. Let me know if you need anything else!