Only import functions from windows.h when building on Windows.

Prevents build failures like

    Linking dist-ghc/build/doctests/doctests ...
    dist-ghc/build/doctests/doctests-tmp/Main.o: In function `s5aC_info':
    (.text+0xb7): undefined reference to `SetConsoleCP'
    dist-ghc/build/doctests/doctests-tmp/Main.o: In function `r57z_info':
    (.text+0x1c6): undefined reference to `GetConsoleCP'
    dist-ghc/build/doctests/doctests-tmp/Main.o: In function `s5dq_info':
    (.text+0x366): undefined reference to `SetConsoleCP'
    dist-ghc/build/doctests/doctests-tmp/Main.o: In function `s5dr_info':
    (.text+0x42d): undefined reference to `SetConsoleCP'
    dist-ghc/build/doctests/doctests-tmp/Main.o: In function `s5dv_info':
    (.text+0x522): undefined reference to `GetConsoleCP'
    collect2: error: ld returned 1 exit status