COMP: add wmake rules for mingw (#1238)

doc/cross-compile-mingw.md

@@ -0,0 +1,131 @@
+Notes for cross-compiling with mingw.
+
+On openSUSE use the packages:
+```
+mingw64-cross-binutils
+mingw64-cross-cpp
+mingw64-cross-gcc
+mingw64-cross-gcc-c++
+mingw64-filesystem
+mingw64-headers
+mingw64-runtime
+
+mingw64-libwinpthread1
+mingw64-winpthreads-devel
+
+mingw64-libfftw3
+mingw64-fftw3-devel
+```
+
+This setup is missing zlib, so download that manually and compile as a
+*static* library.
+
+```
+CC="$(wmake -show-c) CFLAGS="$(wmake -show-cflags) ./configure --static
+```
+
+The resulting output files (zconf.h zlib.h) and (libz.a) either need
+to be installed in system locations where OpenFOAM can find it, or if
+they are to be shipped directly with OpenFOAM, they can also be placed
+in the `src/OpenFOAM/include` and `platforms/XXX/lib` paths.
+
+When using the cross-compiled executables and libraries, additional
+runtime libraries are required. On openSUSE these are provided by the
+packages:
+```
+mingw64-libgcc_s_seh1
+mingw64-libstdc++6
+```
+
+In a few locations within OpenFOAM, flex is used to generate code. The
+generated C++ code requires the `FlexLexer.h` header file.
+This is normally located under `/usr/include/FlexLexer.h`, which will
+be ignored by the cross-compiler.
+
+As a fairly ugly hack, a copy of this file can be made in a standard
+project include location. For example,
+
+```
+mkdir src/OpenFOAM/lnInclude
+cp /usr/include/FlexLexer.h src/OpenFOAM/lnInclude
+```
+
+
+The last point to consider when cross-compiling is the behaviour of
+the OpenFOAM tools used during compilation. These are found under
+`wmake/src`. When these are compiled they will create executables that
+work on the target platform (Windows), but *not* on the host platform.
+
+The workaround:
+
+1. Activate the native OpenFOAM environment (Linux with system gcc)
+and use that to compile the build tools
+```
+wmake/src/Allmake
+```
+This can be skipped if you already have an existing OpenFOAM build.
+
+2. Activate the OpenFOAM for cross-compiling (Linux with mingw)
+and use that to compile the build tools
+```
+wmake/src/Allmake
+```
+
+3. Copy the contents of the native platform build tools into the
+cross-compilation platform directory. For example,
+```
+cp wmake/platforms/linux64Gcc/* wmake/platforms/linux64Mingw/
+```
+
+The `wmake/platforms/linux64Mingw/` directory should now contain
+native and cross-compiled tools:
+```
+dirToString
+wmkdep
+wmkdepend
+
+dirToString.exe
+wmkdep.exe
+wmkdepend.exe
+```
+
+The native tools are the one that will (automatically) be used
+throughout the balance of the cross-compilation process.
+
+
+Adjust paths for third-party sources. Eg, in `etc/config.sh/FFTW` you
+may wish to have the following:
+```
+fftw_version=fftw-system
+export FFTW_ARCH_PATH=/usr/x86_64-w64-mingw32/sys-root/mingw
+```
+
+
+The settings for cross-compilation are normally defined in the
+`etc/pref.sh` file with contents like this:
+
+```
+# For mingw cross-compile
+
+export WM_COMPILER=Mingw
+export WM_MPLIB=MSMPI
+
+export WM_LABEL_SIZE=32
+# other settings...
+```
+
+
+Additional adjustments may be required in some other places. For example
+in `etc/config.sh/FFTW`
+```
+fftw_version=fftw-system
+export FFTW_ARCH_PATH=/usr/x86_64-w64-mingw32/sys-root/mingw
+```
+
+
+Known limitations (2019-05-01)
+
+- No CGAL support (ie, no foamyHexMesh)
+- No ParaView plugin, runTimepostProcessing
+- reacting EulerFoam solvers have too many interdependencies and do
+  not yet compile.