Adding an optional meson support is a good idea (generally), at least on POSIX, as it is a modern and very easy to use (yet very capable) build system.
However, I think it might be better to expose a generic builder api in TheIDE/umk for making build tool scripts... (instead of a hardcoded make/cmake script creator we have...)
And this can and should be done with TheIDE/umk + Esc-based macros (then it can be implemented and shipped as different "build-script maker" macros without a costly maintenance burden or broken code in theIDE's code base...
This way, we can even collect different build script creator macros into a single, externally curated package (say, in UppHub)