python3-meson-opt (meson installed to /opt)

Meson is a cross-platform build system designed to be both as fast and
as user friendly as possible. It supports many languages and
compilers, including GCC, Clang, PGI, Intel, and Visual Studio. Its
build definitions are written in a simple non-Turing complete DSL.

NOTE: 
  Install bin and docs to /opt/meson
  Installs module to /opt/python3.9/site-packages

Add the following before the python3 build commands in your
Slackbuild to use this version over the stock one in 15.0.  My testing
has shown the newer meson bin does not need to be added to PATH, but
YMMV:

export PYTHONPATH=/opt/python3.9/site-packages/