Visual Studio 2015 doesn't have cl.exe
Solution 1:
Visual Studio 2015 doesn't install C++ by default. You have to rerun the setup, select Modify and then check Programming Language -> C++
Solution 2:
In Visual Studio 2019 you can find cl.exe inside
32-BIT : C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.20.27508\bin\Hostx86\x86
64-BIT : C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.20.27508\bin\Hostx64\x64
Before trying to compile either run vcvars32 for 32-Bit compilation or vcvars64 for 64-Bit.
32-BIT : "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars32.bat"
64-BIT : "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat"
If you can't find the file or the directory, try going to C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC
and see if you can find a folder with a version number. If you can't, then you probably haven't installed C++ through the Visual Studio Installation yet.