Document cross-compile from Linux to Windows.

This uses the mingw32 tool chain on Linux to build Windows
binaries.

mingw-cross.txt

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+
+These are instructions for building Icarus Verilog binaries for
+Windows using mingw cross compiler tools on Linux.
+
+To start with, you need the mingw64-cross-* packages for your linux
+distribution, which gives you the x86_64-w64-mingw32-* commands
+installed on your system. Installing the cross environment is outside
+the scope of this writeup.
+
+First, configure with this command:
+
+  $ ./configure --host=x86_64-w64-mingw32
+
+This generates the Makefiles needed to cross compile everything with
+the mingw32 compiler. The configure script will generate the command
+name paths, so long as commands line x86_64-w64-mingw32-gcc
+et. al. are in your path.
+
+Next, compile with the command:
+
+  $ make HOSTCC=cc HOSTCFLAGS=-O
+
+The configure generated the cross compiler flags, but there are a few
+bits that need to be compiled with the native compiler. (version.exe
+for example is used by the build process but is not installed.) The
+MOSTCC= flag on the make command line makes sure the Makefile knows
+about the native tool.