Okay, so I tried that, but the C++ package was already existent. I did update Visual Studio Community, but it gives me the same error.
Also, the update added vcvarsall.bat, which was previously not in my installation, so I tried that with the Python build option, which gave me the same error as previously:
./ninja>python configure.py --bootstrap
warning: A compatible version of re2c (>= 0.11.3) was not found; changes to src/*.in.cc will not affect your build.
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.28.29910\include\cstdio(12): fatal error C1083: Cannot open include file: 'stdio.h': No such file or directory