qmake fails to generate a visual studio project

I use this command to generate a visual studio project from a .pro file:

C:\Qt\5.9.0-win32\5.9\msvc2015\bin\qmake.exe -tp vc myproject.pro

 or this one:

C:\Qt\5.9.0-win32\5.9\msvc2015\bin\qmake.exe -spec win32-msvc -tp vc myproject.pro

Both fail equally with the following errors:

C:/Qt/5.9.0-win32/5.9/msvc2015/mkspecs/features/toolchain.prf:129: Variable QMAKE_CXX.COMPILER_MACROS is not defined.
Project ERROR: msvc-version.conf loaded but QMAKE_MSC_VER isn't set

In Qt 5.9.1 it works better, but the following issues still persist:

1) qmake -spec win32-msvc -tp vc test.pro works from the Visual Studio Developer Command Prompt only. When run from the command line it gives this error: 

"Project ERROR: Cannot run compiler 'cl'. Maybe you forgot to setup the environment?"

Setting VS environment variables by running vcvarsall.bat may help in this case, but previously it wasn't needed.

2) The generated VS project file is missing Debugging -> Environment setting, hence the project may not work properly. For instance, the attached sample project fails to load the image from the resource file. In Qt Creator the same project works fine. Once the Environment has been set, it works in Visual Studio too. In my case I had to manually set it to "PATH=C:\Qt\5.9.1\5.9.1\msvc2015\bin;$(PATH)".