switching remaining platforms to modular X by default

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

The latest pango version needs a thread-safe fontconfig, i.e. version
2.10.91 or newer.

For users of "native" X on NetBSD, which provides an older version,
this currently leads to breakage because two versions of fontconfig
are linked in.

This is just the latest case of native X support causing hard-to-fix
problems. For another example, harfbuzz is broken on NetBSD-5.0.x for
months due to a freetype issue (at least since January, I didn't check
older logs).

Since supporting multiple versions of libraries is hard, and no one is
taking care of it in the case of native X even for supported
platforms, I suggest we switch the default for the remaining platforms
that currently default to 'native' to 'modular'. This will affect
Darwin, NetBSD, OpenBSD, Minix, MirBSD, SunOS (with installed X
headers), and Cygwin.

I understand that this will cause additional build time and disk space
in some cases; so I don't suggest removing the support for 'native'
completely, so people who care about this can still use it (as far as
it works).

Comments?
 Thomas