Fix building on MSVC

[FtZPetruska]

While working on #19, I noticed building on MSVC due to the following problems:

• __attribute__((unused)) does not exist, this is only used in some files from unzip.
• VLAs are not supported, this is used once in statesaver.cpp:553.

For the __attribute__ issue, I just added a simple check for MSVC:

#ifdef _MSC_VER
#define ATTRIBUTE_UNUSED
#else
#define ATTRIBUTE_UNUSED __attribute__((unused))
#endif

For the VLA, I simply replaced it with a std::string as it provides a convenient way to replicate the behaviour.

[CasualPokePlayer]

Unused parameters probably are better off using the standard way indicate them, with (void)unused_variable;.

I'm not sure if std::string is appropriate here, as this is all raw data rather than some string. A std::unique_ptr<char[]> may be better here.