mingw: set _WIN32_WINNT explicitly for Git for Windows

Previously, we only ever declared a target Windows version if compiling with Visual C. Which meant that we were relying on the MinGW headers to guess which Windows version we want to target... Let's be explicit about it, in particular because we actually want to bump the target Windows version to Vista (which we will do in the next commit). Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>