Dropping MinGW32 support in favor of MinGW-w64

[mosra]

MinGW32 has several issues and it seems that the developers don't care about the users anymore, for example:

• completely missing std::to_string() functions, which I need to work around using slow std::stringstream. The related bug was closed as "out of date" and no fix is in sight.
• complete lack of testing -- GCC 4.8.1 being totally unusable (the executables crashed on execution), while GCC 4.7 includes needed to be patched to make them work with C++11. I even had to create a troubleshooting page on Corrade's wiki to address these issues. Not sure whether this got fixed already.
• completely broken std::u32string on GCC 4.7 (memory corruption on destruction), so I had to workaround it everywhere by using std::vector<char32_t> instead. Again, not sure whether this works now.
• complete lack of 64-bit support, which is shameful.

As ArchLinux maintainers removed all MinGW32 packages and replaced them with MinGW-w64 equivalents, I think it would be good to do the same, clean up all these ugly MinGW32-specific workarounds and support just MinGW-w64.

Is there anyone using MinGW32 who can't switch to MinGW-w64? These cases come to my mind:

• old or obscure Linux distributions not having MinGW-w64 in repositories (probably not)
• Windows toolchains which come with MinGW32 bundled but without ability to replace it with anything else (in Qt SDK it is possible to replace it, as far as I know)

[mosra]

Apparently all mentioned issues are fixed with MinGW-w64 GCC 5.1, removed the workarounds in corrade, magnum and magnum-plugins.

Closing as done.