add support file for cross-compiling Windows library

Windows64.cmake

+# Cross-compile for Windows (64-bit) using Mingw-w64
+
+# Build using:
+#   cmake . -Bbuild -DCMAKE_TOOLCHAIN_FILE=Windows64.cmake
+#   cmake --build build
+
+# from @ticho:cyberdi.sk
+# https://paste.debian.net/1201338/
+
+# the name of the target operating system
+SET(CMAKE_SYSTEM_NAME Windows)
+
+# which compilers to use for C and C++
+SET(CMAKE_C_COMPILER x86_64-w64-mingw32-gcc)
+SET(CMAKE_CXX_COMPILER x86_64-w64-mingw32-g++)
+SET(CMAKE_RC_COMPILER x86_64-w64-mingw32-windres)
+
+# here is the target environment located
+SET(CMAKE_FIND_ROOT_PATH /usr/x86_64-w64-mingw32)
+
+# adjust the default behaviour of the FIND_XXX() commands:
+# search headers and libraries in the target environment, search
+# programs in the host environment
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+
+# static-link against the standard libraries
+set(CMAKE_CXX_STANDARD_LIBRARIES "-static-libgcc -static-libstdc++")