cmake: new toolchain file for MinGW cross-compilation on Ubuntu

cmake/toolchain/Ubuntu20_04_mingw-w64-x86_64.cmake

@@ -0,0 +1,25 @@
+# Sample toolchain file for building for Windows from an Ubuntu 20.04 Linux system.
+#
+# Typical usage:
+#    *) install cross compiler: `sudo apt-get install mingw-w64`
+#    *) install zlib for mingw: sudo apt install libz-mingw-w64-dev
+#    *) mkdir buildMinGWRelease && cd buildMinGWRelease
+#    *) cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchain/Ubuntu20_04_mingw-w64-x86_64.cmake 
+#        -DEXIV2_ENABLE_XMP=OFF -DEXIV2_TEAM_EXTRA_WARNINGS=ON -DEXIV2_ENABLE_WIN_UNICODE=ON
+
+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)