Compile with `-D_WIN32_WINNT=0x501` to use a WinXP-compatible API.
Compile with `-D_USING_V110_SDK71_` to tell the VS standard library
headers that we are building with a WinXP-compatible Windows SDK. Link
executables with `-subsystem:console,5.01` to make them runnable on
Windows XP. Ideally `cmake-gui` should instead be linked with
`-subsystem:windows,5.01` but with the Ninja and Makefile generators
CMake adds `-subsystem:windows` after our `-subsystem:console,5.01` flag
and the linker seems to interpret this combination as we need.
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