meson --prefix & --mandir ignored

[Geremia]

I'm trying to package it for Slackware Linux:

meson setup --prefix=/usr --mandir=/usr/man build
meson compile -C build -j$(nproc)
meson test -C build
meson install -C build --destdir $PKG

But meson setup ignores the --prefix and --mandir options and puts the files in /usr/local/share and /usr/local/share/man instead. Is there a way to correctly specify the prefix and mandir directories?

Thank you for continuing to maintain the great n-way diff program!