Add the capability to build univalue using cmake/ninja

Summary: This is much better than autotool. Hopefully, over time, we'll be able to convert the rest of the build and use that.

Test Plan:

mkdir build
cd build
cmake -GNinja ..
ninja

And cross compil for windows:

mkdir buildwin32
cd buildwin32
cmake -GNinja -DCMAKE_TOOLCHAIN_FILE=../cmake/plateforms/Win32.cmake
ninja

Repeat for win64, which should be obvious.

Reviewers: schancel, #bitcoin_abc, jasonbcox

Reviewed By: schancel, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D773