>>949を実行したら
$ meson builddir -Dgdk_pixbuf_query_loaders_path=/usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/gdk-pixbuf-query-loaders
The Meson build system
Version: 0.49.2
Source dir: /tmp/webp-pixbuf-loader
Build dir: /tmp/webp-pixbuf-loader/builddir
Build type: native build
meson.build:1:0: ERROR: Meson version is 0.49.2 but project requires >=0.50.
A full log can be found at /tmp/webp-pixbuf-loader/builddir/meson-logs/meson-log.txt
と出たので Mesonのバージョンが0.50.以上必要でダメということですかね。う〜ん残念。