Fix the build on MinGW x64

[jpcima]

Expected to fix #34. Not actually tested, but it builds.

• the definition __LP64__ is not sufficient to check for 64-bit, because MinGW does not have it; it's edited to support the MinGW case, as seen in other places.
• adapt CMake to build with nasm in case of MinGW (uses variable MINGW, added in CMake 3.2)