meson.build: Support ncurses on MacOS and OpenBSD

MacOS provides header files for curses 5.7 with support
for wide characters, but requires _XOPEN_SOURCE_EXTENDED=1
to activate that.

By default those old header files are used even if there
is a newer Homebrew installation of ncurses 6.2 available.

Change also the old macro definition of NCURSES_WIDECHAR
and set it to 1 like it is done in newer versions of
curses.h when _XOPEN_SOURCE_EXTENDED=1 is defined.

OpenBSD has the same version of ncurses and needs the same fix.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Brad Smith <brad@comstyle.com>
Message-Id: <20211117205355.1392292-1-sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>