cmake toolchain file for mingw-w64 x86_64 builds on Ubuntu

mingw-w64-x86_64.cmake

# Sample toolchain file for building for Windows from an Ubuntu Linux system.
#
# Typical usage:
#    *) install cross compiler: `sudo apt-get install mingw-w64`
#    *) cd build
#    *) cmake -DCMAKE_TOOLCHAIN_FILE=~/mingw-w64-x86_64.cmake ..
# This is free and unencumbered software released into the public domain.

set(CMAKE_SYSTEM_NAME Windows)
set(TOOLCHAIN_PREFIX x86_64-w64-mingw32)

# cross compilers to use for C, C++ and Fortran
set(CMAKE_C_COMPILER ${TOOLCHAIN_PREFIX}-gcc)
set(CMAKE_CXX_COMPILER ${TOOLCHAIN_PREFIX}-g++)
set(CMAKE_Fortran_COMPILER ${TOOLCHAIN_PREFIX}-gfortran)
set(CMAKE_RC_COMPILER ${TOOLCHAIN_PREFIX}-windres)

# target environment on the build host system
set(CMAKE_FIND_ROOT_PATH /usr/${TOOLCHAIN_PREFIX})

# modify default behavior of FIND_XXX() commands
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

@peterspackman Thank you so much for your contribution and for taking the time to do that!

If using pkg-config, you also need to do something like set(PKG_CONFIG_EXECUTABLE ${TOOLCHAIN_PREFIX}-pkg-config) so that FindPkgConfig searches for cross compiled packages, not those for running where cmake is running.

I had dependencies also checking for CMAKE_SYSTEM_PROCESSOR, I believe this line should be added:

set(CMAKE_SYSTEM_PROCESSOR x86_64)

This is immensely helpful. Thank you so much.