`bc` command is not available on Windows

[przemyslaw-zan]

bc command used here is not available on Windows, and causes the script to throw an error.

https://github.com/ckeditor/ckeditor4-presets/blob/0810ebc3383c42289085e7b19772bcb814c46765/build.sh#L148

[przemyslaw-zan]

Some additional context: I was using bash that comes with Git for Windows.

The fix for windows is as follows:

• manually download the archive from msys2.org
• unpack the archive (for example using 7-zip) - notice that it is packed twice: .tar.xz
• place the usr/bin/bc.exe file in directory available to PATH environment variable

bc should now be available within bash.