-e: see title. even an empty build fails on Windows and OSX.

-tested on Version 5.4.0b3 (6110a3bcdf6e), Tue, 26 Jan 2016 23:11:05 GMT, Branch: trunk. Worked on the previous beta build.

-repro:
--create new project
--start a webgl build for the empty project
--NOTICE it fails with IL2CPP errors

Failed running D:\Unity Installs\5.4.0b4\Editor\Data\il2cpp/build/il2cpp.exe --convert-to-cpp --copy-level=None --output-format=Compact --compile-cpp --platform="WebGL" --architecture="EmscriptenJavaScript" --configuration="Release" --outputpath="D:/Work/Bugs/New Unity Project 36/Assets /../Temp/StagingArea/Data\Native\UserAssembly.bc" --cachedirectory="D:/Work/Bugs/New Unity Project 36/Assets/../Library/il2cpp_cache" --compiler-flags="-Oz -DIL2CPP_EXCEPTION_DISABLED=1 " --linker-flags="-Oz -s NO_EXIT_RUNTIME=1" --additional-include-directories="D:/Work/Bugs/New Unity Project 36/Assets/../Temp/StagingArea/Data\Native\UserAssembly.bc" --extra-types.file="D:\Unity Installs\5.4.0b4\Editor\Data\il2cpp\il2cpp_default_extra_types.txt" --assembly="D:\Work\Bugs\New Unity Project 36\Temp\StagingArea\Data\Managed\UnityEngine.dll" --generatedcppdir="D:\Work\Bugs\New Unity Project 36\Temp\StagingArea\Data\il2cppOutput"

stdout:
stderr:

Unhandled Exception: System.TypeInitializationException: The type initializer for 'Unity.IL2CPP.Building.EmscriptenPaths' threw an exception. ---> System.InvalidOperationException: Parent is called on an empty path
at NiceIO.NPath.get_Parent()
at Unity.IL2CPP.Building.EmscriptenPaths.get_UnityRoot()
at Unity.IL2CPP.Building.EmscriptenPaths.get_WebGlRoot()
at Unity.IL2CPP.Building.EmscriptenPaths.get_EmscriptenConfig()
at Unity.IL2CPP.Building.EmscriptenPaths..cctor()
--- End of inner exception stack trace ---
at Unity.IL2CPP.Building.EmscriptenPaths.get_Python()
at Unity.IL2CPP.Building.EmscriptenToolChain.CompilerExecutableFor(NPath sourceFile)
at Unity.IL2CPP.Building.CppProgramBuilder.ProvideObjectFile(CppSourceCompileInstruction cppSourceCompileInstruction)
at Unity.IL2CPP.Building.ParallelFor.<RunWithResult>c__AnonStorey1`2.<>m__0(Object o)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart(Object obj)

Unhandled Exception:

UnityEngine.Debug:LogError(Object)
UnityEditorInternal.Runner:RunManagedProgram(String, String, String, CompilerOutputParserBase, Action`1) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:98)
UnityEditorInternal.IL2CPPBuilder:ConvertPlayerDlltoCpp(ICollection`1, String, String) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:351)
UnityEditorInternal.IL2CPPBuilder:Run() (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:213)
UnityEditorInternal.IL2CPPUtils:RunIl2Cpp(String, IIl2CppPlatformProvider, Action`1, RuntimeClassRegistry, Boolean) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:139)
UnityEditor.HostView:OnGUI()