copy all autotools compiler warnings to meson build

There a large number of differences in the compiler warnings between the meson and autotools builds. This is my attempt to port across all the warnings defined in build/configure.ac.warnings to meson.

I've also added -Wno-enum-conversion to silence all the cairo_status_t to cairo_int_status_t warnings.

Building with meson now produces a useful set of warnings that can be further investigated.