Hxcpp thinks it's on Windows in Bash on Windows

Running `compile-cpp.hxml` of the Haxe repo's unit tests, for some reason `-DHX_WINDOWS` is defined and it's trying to use `x86_64-w64-mingw32-g++` instead of the regular `g++` which is installed. Works just fine in my Virtual Machine, which is also an Ubuntu 14.04. Also, both were on the same Haxe dev build (cbee676) and the same hxcpp version (both use the haxelib folder on my Windows file system).

```
haxelib run hxcpp Build.xml haxe -Ddebug -DHXCPP_STACK_LINE="1" -DHXCPP_STACK_TRACE="1" -DHXCPP_VERBOSE="1" -Danalyzer="1" -Danalyzer-code-motion="1" -Danalyzer-user-var-fusion="1" -Danalyzer_code_motion="1" -Danalyzer_user_var_fusion="1" -Dhaxe3="1" -Dhaxe_ver="3.3" -Dhxcpp_api_level="330" -Dmacro-times="1" -Dmacro_times="1" -Dsource-header="Generated by Haxe 3.3.0 (git build development @ cbee676)" -I"C:\\Program Files\\The Haxe Effect\\src/dev/null/" -I"src/" -I"" -I"/home/jens/haxe/std/cpp/_std/" -I"/home/jens/haxe/std/"
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