w32: Always use UTF-8 for localized texts

* configure.ac: Define GPG_ERR_ENABLE_GETTEXT_MACROS.
* src/version.c (do_subsystem_inits) [W32]: Switch gettext to UTF-8.

On Windows, applications using GpgME can now rely on localized texts
returned by GpgME, e.g. the texts for error codes, to be UTF-8 encoded.
In particular, this saves a useless and possibly lossy conversion to
native encoding and back to UTF-8. On other OSes, where we use the
system provided gettext, we assume that UTF-8 is used nowadays.

• GnuPG-bug-id: T5960