build: Require va_copy() or __va_copy() on non-MSVC compilers

va_copy() is a C99 feature, and should be widely supported by now.
gcc in strict C89 mode implements an equivalent __va_copy() instead.

MSVC 2013 implements va_copy(), but at the moment we still aim to support
MSVC 2010 and 2012, which don't have it. However, we know that in
Windows ABIs, va_list is a pointer, so we can use
_DBUS_VA_COPY_ASSIGN. We do not support MSVC for Autotools builds, only
CMake, due to its non-Unixish command-line interface.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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