Support building using the latest mingw-w64 toolchain.
* acinclude.m4 (GNUPG_SYS_SYMBOL_UNDERSCORE): Change mingw detection.
This patch is related to Debian-bug-id 730271 for GnuPG 1.4:
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.
- Signed-off-by: Werner Koch <wk@gnupg.org>