build: Add the remaining -Wextra warnings in GCC 8 to the Meson build
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.