add OS-specific build notes

README

@@ -69,3 +69,19 @@ install the package on most systems.
   ./configure   # see ./configure --help for configuration options
   make check    # runs builtin unit tests
   make install  # set DESTDIR= to install to an alternate location
+
+OS specific build information:
+
+ - HP-UX (11i)
+   Set the UNIX_STD environment variable to '2003' before running 'configure'
+   in order to build with the HP C/aC++ compiler. See the "standards(5)" man
+   page for more details.
+
+    export UNIX_STD=2003
+    ./configure
+    make
+
+ - Windows - Mingw-w64
+   LibreSSL builds against relatively recent versions of Mingw-w64, not to be
+   confused with the original mingw.org project.  Mingw-w64 3.2 or later
+   should work. See README.windows for more information

README.windows

@@ -36,5 +36,7 @@ cv2pdb to generate Visual Studio and windbg compatible debug files. cv2pdb is a
 tool developed for the D language and can be found here:
 https://github.com/rainers/cv2pdb
 
-Pre-build Windows binaries are available with the LibreSSL release for your
-convenience.
+Pre-built Windows binaries are available with LibreSSL releases if you do not
+have a mingw-w64 build environment. Mingw-w64 code is largely, but not 100%,
+compatible with code built from Visual Studio. Notably, FILE * pointers cannot
+be shared between code built for Mingw-w64 and Visual Studio.