Replace the commit message with this one:
Assorted header fixes to 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).
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>