appveyor: Use two parallel compilation on appveyor with CMake

Appveyor provides 2 CPUs for each builder[1], make sure to use parallel
compilation, when running with CMake. CMake learned this new option in
version 3.12[2] and the version provided by appveyor is fresh enough.

Curl doesn't really take that long to build and it is using the slowest
builder available, msbuild, so expect only a moderate improvement in
build times.

[1] https://www.appveyor.com/docs/build-environment/
[2] https://cmake.org/cmake/help/v3.12/release/3.12.html

Closes #4508

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