我在编译assimp android时找不到工具链文件：$ANDTOOLCHAIN。
Error:
PS E:\assimp-master> cd buildandroid
PS E:\assimp-master\buildandroid> cmake -DCMAKE_TOOLCHAIN_FILE=$ANDTOOLCHAIN -DENABLE_BOOST_WORKAROUND=ON -DCMAKE_INSTAL
L_PREFIX=/assimp-2.0 -DANDROID_ABI=armeabi-v7a _DANDROID_NATIVE_API_LEVEL=android-8 -DANDROID_FORCE_ARM_BUILD=TRUE ..
-- Building for: Visual Studio 15 2017
CMake Error at C:/Program Files/CMake/share/cmake-3.12/Modules/CMakeDetermineSystem.cmake:100 (message):
Could not find toolchain file: $ANDTOOLCHAIN
Call Stack (most recent call first):
CMakeLists.txt:39 (PROJECT)
CMake Error at CMakeLists.txt:39 (PROJECT):
Failed to run MSBuild command:
C:/Program Files (x86)/Microsoft Visual Studio/2017/Professional/MSBuild/15.0/Bin/MSBuild.exe
获取VCTargetsPath的值：
Microsoft (R) Build Engine version 15.8.168+ga8fba1ebd7 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.
Build started 07-09-2018 11:08:04.
Project "E:\assimp-master\buildandroid\CMakeFiles\3.12.1\VCTargetsPath.vcxproj" on node 1 (default targets).
C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\IDE\VC\VCTargets\Microsoft.Cpp.WindowsSDK.targets(46,5): error MSB8036: The Windows SDK version 8.1 was not found. Install the required version of Windows SDK or change the SDK version in the project property pages or by right-clicking the solution and selecting "Retarget solution". [E:\assimp-master\buildandroid\CMakeFiles\3.12.1\VCTargetsPath.vcxproj]
Done Building Project "E:\assimp-master\buildandroid\CMakeFiles\3.12.1\VCTargetsPath.vcxproj" (default targets) -- FAILED.
Build FAILED.
"E:\assimp-master\buildandroid\CMakeFiles\3.12.1\VCTargetsPath.vcxproj" (default target) (1) ->
(_CheckWindowsSDKInstalled target) ->
C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\IDE\VC\VCTargets\Microsoft.Cpp.WindowsSDK.targets(46,5): error MSB8036: The Windows SDK version 8.1 was not found. Install the required version of Windows SDK or change the SDK version in the project property pages or by right-clicking the solution and selecting "Retarget solution". [E:\assimp-master\buildandroid\CMakeFiles\3.12.1\VCTargetsPath.vcxproj]
0 Warning(s)
1 Error(s)
Time Elapsed 00:00:01.38
退出代码：1
-- Configuring incomplete, errors occurred!