meson.build: Fix mingw format specifiers

On Windows, format specifiers like "%llu"
will issue a warning for 64bit integers
on ancient MINGW compilers.
To mitigate this problem, define
__USE_MINGW_ANSI_STDIO =1  can be used
to modify stdio.h behaviour
8 jobs for fix-mingw-format-specifiers in 20 minutes and 4 seconds (queued for 1 minute and 6 seconds)