MSYS2 - compilation on Windows

[syzygy1]

Today I installed MSYS2 on a Windows laptop and had some problems getting Cfish to compile (due to _WIN32 not being defined) until I found out that I had to install the MinGW 64-bit toolchain. I have therefore added some very basic instructions to the README. If someone happens to try them out and finds that more packages need to be installed or that some other crucial information is missing, please let me know.

[ppigazzini]

Some notes:

• the package group mingw-w64-x86_64-toolchain installs all the gnu programming packages (ada. fortran, objc, gdb etc.). The minimal setup is:
  pacman -S mingw-w64-x86_64-gcc mingw-w64-x86_64-make
• both previous setups install mingw32-make, so the directive to compile Cfish is:
  mingw32-make target [ARCH=arch] [COMP=compiler]
  this can be hard to find or confusing for a non expert user of MSYS2/MinGW-w64
• MSYS2 has also the make package that runs with the POSIX emulation layer, in this case the minimal setup is:
  pacman -S mingw-w64-x86_64-gcc make
  and the directive to compile Cfish is the common:
  make target [ARCH=arch] [COMP=compiler]