Rename README.windows to README.mingw.md

Since it is mingw specific and does not really
involve other compilers.

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+## Building with mingw-w64 for 32- and 64-bit
+
+For Windows systems, LibreSSL supports the mingw-w64 toolchain, which can use
+GCC or Clang as the compiler. Contrary to its name, mingw-w64 supports both
+32-bit and 64-bit build environments. If your project already uses mingw-w64,
+then LibreSSL should integrate very nicely. Old versions of the mingw-w64
+toolchain, such as the one packaged with Ubuntu 12.04, may have trouble
+building LibreSSL. Please try it with a recent toolchain if you encounter
+troubles. Cygwin provides an easy method of installing the latest mingw-w64
+cross compilers on Windows.
+
+To configure and build LibreSSL for a 32-bit system, use the following
+build steps:
+
+ CC=i686-w64-mingw32-gcc CPPFLAGS=-D__MINGW_USE_VC2005_COMPAT \
+ ./configure --host=i686-w64-mingw32
+ make
+ make check
+
+For 64-bit builds, use these instead:
+
+ CC=x86_64-w64-mingw32-gcc ./configure --host=x86_64-w64-mingw32
+ make
+ make check
+
+### Why the -D__MINGW_USE_VC2005_COMPAT flag on 32-bit systems?
+
+An ABI change introduced with Microsoft Visual C++ 2005 (also known as
+Visual C++ 8.0) switched time_t from 32-bit to 64-bit. It is important to
+build LibreSSL with 64-bit time_t whenever possible, because 32-bit time_t
+is unable to represent times past 2038 (this is commonly known as the
+Y2K38 problem).
+
+If LibreSSL is built with 32-bit time_t, when verifying a certificate whose
+expiry date is set past 19 January 2038, it will be unable to tell if the
+certificate has expired or not, and thus take the safe stance and reject it.
+
+In order to avoid this, you need to build LibreSSL (and everything that links
+with it) with the -D__MINGW_USE_VC2005_COMPAT flag. This tells mingw-w64 to
+use the new ABI.
+
+64-bit systems always have a 64-bit time_t and are not affected by this
+problem.