The Autotools build uses -Wextra because it's on the AX_COMPILER_FLAGS
list. However, using -Wextra directly is problematic because newer
compilers can add more warnings to the -Wextra umbrella, and some of
them might not be fit for this codebase. eg., GCC 8 added
-Wcast-function-type, which can't be sanely used with the GNOME
platform (see commit bb290a1e). This is why the Meson build
tries to avoid -Wextra. Instead, it is better to explicitly list out
the desired warnings from the -Wextra umbrella.

The warnings that were explicitly listed out in AX_COMPILER_FLAGS were
already present in the initial port to Meson. This fills in the rest
that weren't explicitly listed but were still enabled by -Wextra.