$ flatpak build ../builder meson --libdir=/app/lib -Denable_tracing=true -Denable_debug=true -Dwith_docs=true -Dwith_channel=flatpak-nightly ..

Meson encountered an error:
The value of the 'libdir' option is '/app/lib' which must be a subdir of the prefix '/usr/local'.
Note that if you pass a relative path, it is assumed to be a subdir of prefix.