"Could NOT find OpenSSL" when opening GitAhead in Qt Creator - qt

Question

I can build GitAhead in a "x64 Native Tools Command Prompt for VS2017" just by following the README. All I have to do is:
Have CMake and ninja.exe in the PATH
Check out the project and submodules
In the VS Command prompt, change to the gitahead\build\release directory and run cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=C:\dev-tools\Qt\5.12.0\msvc2017_64 ../.. twice
It builds to completion and I get a GitAhead.exe.
Now I wondered if I can open the project in Qt Creator. According to this I just have to open CMakeLists.txt in Qt Creator.
At first that didn't work at all. By trial and error I came up with the following environment variables that I have to set in Options > Kits > Environment:
CC=cl
CXX=cl
LIB=C:\Program Files (x86)\Windows Kits\10\Lib\10.0.17763.0\um\x64;C:\dev-tools\VisualStudioBuildTools\VC\Tools\MSVC\14.16.27023\lib\x64;C:\Program Files (x86)\Windows Kits\10\Lib\10.0.17763.0\ucrt\x64
PATH=${PATH};C:\Program Files (x86)\Windows Kits\10\bin\10.0.17763.0\x64;C:\dev-tools\VisualStudioBuildTools\VC\Tools\MSVC\14.16.27023\bin\Hostx64\x64
(This might not be the correct way, I don't know what I'm doing.)
But now I'm stuck. Now, when I click "Configure project" I get:
Running "C:\dev-tools\CMake\bin\cmake.exe -E server "--pipe=\\.\pipe\{7a503b92-9fbb-4075-960e-6a968505fe74}" --experimental" in C:\Users\andre\AppData\Local\Temp\QtCreator-lyJxiX\qtc-cmake-OgvZcaJK.
Starting to parse CMake project, using: "-DCMAKE_CXX_COMPILER:STRING=", "-DCMAKE_C_COMPILER:STRING=", "-DCMAKE_PREFIX_PATH:STRING=C:/dev-tools/Qt/5.12.0/msvc2017_64", "-DQT_QMAKE_EXECUTABLE:STRING=C:/dev-tools/Qt/5.12.0/msvc2017_64/bin/qmake.exe".
The C compiler identification is MSVC 19.16.27026.1
The CXX compiler identification is MSVC 19.16.27026.1
Check for working C compiler: C:/dev-tools/VisualStudioBuildTools/VC/Tools/MSVC/14.16.27023/bin/Hostx64/x64/cl.exe
Check for working C compiler: C:/dev-tools/VisualStudioBuildTools/VC/Tools/MSVC/14.16.27023/bin/Hostx64/x64/cl.exe -- works
Detecting C compiler ABI info
Detecting C compiler ABI info - done
Detecting C compile features
Detecting C compile features - done
Check for working CXX compiler: C:/dev-tools/VisualStudioBuildTools/VC/Tools/MSVC/14.16.27023/bin/Hostx64/x64/cl.exe
Check for working CXX compiler: C:/dev-tools/VisualStudioBuildTools/VC/Tools/MSVC/14.16.27023/bin/Hostx64/x64/cl.exe -- works
Detecting CXX compiler ABI info
Detecting CXX compiler ABI info - done
Detecting CXX compile features
Detecting CXX compile features - done
Found Git: C:/dev-tools/Git/cmd/git.exe (found version "2.20.1.windows.1")
CMake Warning (dev) at dep/libssh2/libssh2/CMakeLists.txt:52 (option):
Policy CMP0077 is not set: option() honors normal variables. Run "cmake
--help-policy CMP0077" for policy details. Use the cmake_policy command to
set the policy and suppress this warning.
For compatibility with older versions of CMake, option is clearing the
normal variable 'BUILD_SHARED_LIBS'.
This warning is for project developers. Use -Wno-dev to suppress it.
Could NOT find OpenSSL, try to set the path to OpenSSL root folder in the system variable OPENSSL_ROOT_DIR (missing: OPENSSL_CRYPTO_LIBRARY) (found version "1.1.1a")
Could NOT find Libgcrypt (missing: LIBGCRYPT_LIBRARY LIBGCRYPT_INCLUDE_DIR)
Looking for include files windows.h, bcrypt.h
Looking for include files windows.h, bcrypt.h - not found
CMake Error at dep/libssh2/libssh2/src/CMakeLists.txt:170 (message):
No suitable cryptography backend found.
Configuring incomplete, errors occurred!
See also "C:/Users/andre/AppData/Local/Temp/QtCreator-lyJxiX/qtc-cmake-OgvZcaJK/CMakeFiles/CMakeOutput.log".
See also "C:/Users/andre/AppData/Local/Temp/QtCreator-lyJxiX/qtc-cmake-OgvZcaJK/CMakeFiles/CMakeError.log".
CMake Project parsing failed.
I tried adding
OPENSSL_ROOT_DIR=C:\dev-projects\gitahead\dep\openssl\openssl
to the environment variables, but it makes no difference. I have no idea why CMake can find OpenSSL on the command line (in the VS Command Prompt) but not in Qt Creator. How can I debug this?