Support two build systems increase burden on project maintenance, this is the main reason to reject my pull request and is OK. As for GXml, we have both Autotools and Meson. I’m trying to keep both in sync, as soon as a new file is added, but you may forget one or the other.
While I use GXml on my Windows programs, I need to make sure it will work properly out of the box, like Autotools does, before to remove the later.
The main problem about Meson is: it is moving a lot, with new features added in resent versions; so they are not immediately available, i.e. in Windows through MSYS2. Just need to wait until Meson declares a LTS, or something like that, and is included by default in distributions and in MSYS2.
Extra effort to maintain both build systems makes the difference in my Vala development procedure: Code, Code Test, Run Test, Fix Code, Confirm Test Pass. This process have gained a boost in productivity, because Meson compile time and the way to run tests; so less time compiling, means more time Coding.