[RFC/PATCH v1] Towards MinGW(-W64) cross-compilation

This patch series fixes building on modern MinGW and (32bit only yet) MinGW-W64.

*Compilation* tested on:
 - MSVC (via WinGit environment)
 - msysGit environment
 - Linux cross-toolchain i686-pc-mingw32 (4.8.2) with mingw-runtime-3.20.2
 - Linux cross-toolchain i686-w64-mingw32 (4.8.2) with mingw64-runtime-3.1.0

Stuff still required to make Git build with x86_64 MinGW-W64 toolchain:

1. Drop -D_USE_32BIT_TIME_T that was added in fa93bb to config.mak.uname
because time_t cannot be 32bit on x86_64. I haven't yet figured out what
should break if this define is removed (pointers are welcome) and why it was
added in the first place.

2. Stop passing --large-address-aware to linker. I wonder if it does anything
for 32bit MinGW builds.

3. Fix several places with mismatched pointer size casts.

Building it from Gentoo Linux:

MinGW:

  crossdev -t i686-pc-mingw32
  ARCH=x86 emerge-i686-pc-mingw32 -u dev-libs/libiconv sys-libs/zlib net-misc/curl sys-devel/gettext expat
  cd <git>
  make CROSS_COMPILE=i686-pc-mingw32- CC=i686-pc-mingw32-gcc NO_OPENSSL=1 MINGW=1 CURLDIR=/usr/i686-pc-mingw32/usr

MinGW-W64 (32 bit):

  crossdev -t i686-w64-mingw32
  ARCH=x86 emerge-i686-w64-mingw32 -u dev-libs/libiconv sys-libs/zlib net-misc/curl sys-devel/gettext expat
  cd <git>
  make CROSS_COMPILE=i686-w64-mingw32- CC=i686-w64-mingw32-gcc NO_OPENSSL=1 MINGW=1 CURLDIR=/usr/i686-w64-mingw32/usr

Debian/Ubuntu build instructions are WIP (xdeb is non-trivial at all).