Building on MSYS2 fails

[Juanma Barranquero]

  CC       data.o
pdumper.c: In function 'dump_read_all':
pdumper.c:5078:60: error: conversion from 'size_t' {aka 'long long unsigned int'} to 'unsigned int' may change value [-Werror=conversion]
 5078 |       ssize_t chunk = read (fd, (char *) buf + bytes_read, chunk_to_read);
      |                                                            ^~~~~~~~~~~~~
  CC       doc.o
  CC       editfns.o
  CC       callint.o
cc1.exe: some warnings being treated as errors
make[1]: *** [Makefile:401: pdumper.o] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:424: src] Error 2