libasprintf: On mingw, really use our vasprintf function, not mingw_vasprintf.

libasprintf: On mingw, really use our vasprintf function, not mingw_vasprintf.

mingw now has a definition of 'vasprintf' in <stdio.h> that redirects to
mingw_vasprintf, which does not support argument reordering (i.e.
HAVE_POSIX_PRINTF is not defined). Make sure to avoid this function and use
the one defined in our lib-asprintf.c instead.

* gettext-runtime/libasprintf/lib-asprintf.h (asprintf): Define as macro
redirecting to libasprintf_asprintf.
(vasprintf): Define as macro redirecting to libasprintf_vasprintf.
* gettext-runtime/libasprintf/lib-asprintf.c: Define also libasprintf_asprintf
and libasprintf_vasprintf.
* gettext-runtime/libasprintf/autosprintf.cc: Include lib-asprintf.h last.