Makefile: fix _XOPEN_SOURCE redefinition warning
Makefile: fix _XOPEN_SOURCE redefinition warning

Newer versions of ncurses define _XOPEN_SOURCE with 600. We need 700.
This causes a redefinition warning. To fix it, ncurses CFLAGS come
first, then we undefine _XOPEN_SOURCE and then define it with our value.

cglindkamp/dfm