Support building using the latest mingw-w64 toolchain.

* acinclude.m4 (GNUPG_SYS_SYMBOL_UNDERSCORE): Change mingw detection.
--

  From: Stephen Kitt <skitt@debian.org>

  All MinGW targets require underscores when linking. This patch fixes
  acinclude.m4 and the resulting configure so they don't limit the use
  of underscores to the old mingw32msvc targets.

Debian-bug-id: 730271
Signed-off-by: Werner Koch <wk@gnupg.org>