compat/mingw: support MSys2-based MinGW build

compat/mingw: support MSys2-based MinGW build

The excellent MSys2 project brings a substantially updated MinGW
environment including newer GCC versions and new headers. To support
compiling Git, let's special-case the new MinGW (tell-tale: the
_MINGW64_VERSION_MAJOR constant is defined).

Note: this commit only addresses compile failures, not compile warnings
(that task is left for a future patch).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>