i#118517# - Update twain to version 2.1 with support for Apple and UNIX.

Mingw apparently tries to pass as GNUC and WIN32/64 so we detect it early and force it to work like in previous versions.

Cherry-picked from:

e1431bbb

Also contains:

f3011056 - Fix issue with twain header.
1d6adce0 - Minor style fix.

Conflicts:
	twain/inc/twain.h

Change-Id: I67c557b81661bc91e299309ade70d603160b6bca
Reviewed-on: https://gerrit.libreoffice.org/6508Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>