meson: define _FILE_OFFSET_BITS=64 for MinGW. See #1476

The autotools build set it by default and we use off_t in various places,
even on Windows. Also set it with the meson build to avoid any regressions.

Ideally we shouldn't use off_t and use 64bit capable API on Windows instead, so
we get large file support with MSVC as well.
9 jobs for mingw-file-offset-64 in 14 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #78948
cross-android_api21_arm64

00:06:30

passed #78949
cross-android_api28_arm64

00:07:39

passed #78950
cross-mingw64

00:08:40

passed #78956
fedora-x86_64

00:04:27

manual #78953
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #78951
win32
msys2-mingw32

00:08:04

passed #78952
win32
vs2017-x64

00:06:05

failed #78947
fedora-x86_64

00:09:26

 
  Coverage
passed #78954
coverage

00:01:48

73.3%