gfile: Ensure loff_t is defined on FreeBSD

When `copy_file_range()` support was added, I used the definition of
`copy_file_range()` from Linux, which uses `loff_t` to abstract the
different `off*_t` types.

`loff_t` doesn’t exist on FreeBSD, so this doesn’t compile, and was
caught in subsequent asynchronous CI.

Define `loff_t` with a fallback value if it’s not defined, which should
fix this and other uses of `loff_t` in `gfile.c` (for example, if
FreeBSD ever starts declaring `splice()`).

Fixes this CI failure: https://gitlab.gnome.org/GNOME/glib/-/jobs/2812302

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

meson.build

@@ -828,6 +828,10 @@ if cc.has_header_symbol('dlfcn.h', 'RTLD_NEXT', args: '-D_GNU_SOURCE')
   glib_conf.set('HAVE_RTLD_NEXT', 1)
 endif
 
+if cc.has_type('loff_t', prefix: '#include <sys/types.h>')
+  glib_conf.set('HAVE_LOFF_T', 1)
+endif
+
 # Check whether to use statfs or statvfs
 # Some systems have both statfs and statvfs, pick the most "native" for these
 if have_func_statfs and have_func_statvfs