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
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.