From @elextr's comment above:
Correct, as @eli-schwartz said, on Debians and derivatives the "correct" place is the architecture specific directory `lib/x86_64-linux-gnu`, not in basic `lib`. I expect this is because Debian builds for x86_64 and also ARM and IBM and they need to be separated. So in this case the meson build is more correct than the autotools build and should stay as it is.
My comment which is being referred to:
The issue with `$prefix/lib/x86_64-linux-gnu` is because there's a Debian quirk. For both autotools and meson, the default if not specified is `--libdir=lib`, however, only with Meson that default changes on systems where dpkg is installed and the debian release file is found in /etc, to the directory output by `dpkg-architecture -qDEB_HOST_MULTIARCH`.
With autotools, the official debian package for geany instead just passes an option to `./configure --libdir='${prefix}/lib/x86_64-linux-gnu .....` because they really, really, really want all projects to use that directory.
The user specified value `--libdir=lib` will always override that. But apparently it's considered a bad idea on Debian.