Perl's XSUB.h header defines macros to replace libc functions.  Our header
port_win32.h does something similar earlier, so XSUB.h causes compiler
warnings about macro redefinition.  Undefine our macros before including
XSUB.h.

Thomas Munro

Discussion: https://postgr.es/m/CAEepm%3D3%3DTDYEXUEcHpEx%2BTwc31wo7PA0oBAiNt6sWmq93MW02A%40mail.gmail.com