Use xmake instead of cmake

[da-liii]

See https://github.com/XmacsLabs/mogan/pull/544

We are migrating Mogan from cmake to xmake.

Using xmake, we do not need to include the source code of the Hummus PDF Writer.

Just:

add_requires("pdfhummus 4.1",{system=false,configs={libpng=true,libjpeg=true}})

Here is the package definition for pdfhummus contributed by @jingkaimori.

https://github.com/xmake-io/xmake-repo/blob/master/packages/p/pdfhummus/xmake.lua