MinGW and WinXP support (issue #102) (#103)

* Add MinGW and WinXP support, remove Win SDK dep when using MinGW

Was meant to add MinGW support only, WinXP support was made by accident.
Changes:
* Remove Win SDK dependency when compiled with MinGW
* Remap the Win SDK-depended functions to string.h substitutes
* Remap missing WinAPI call RegSetKeyValueW to a substitute function

* Remove warnings by pragma when using MinGW

src/serialization.h

@@ -2,6 +2,7 @@
 
 #include <stdint.h>
 
+#ifndef __MINGW32__
 #pragma warning(push)
 
 #pragma warning(disable : 4061) // enum is not explicitly handled by a case label
@@ -9,12 +10,15 @@
 #pragma warning(disable : 4464) // relative include path contains
 #pragma warning(disable : 4668) // is not defined as a preprocessor macro
 #pragma warning(disable : 6313) // Incorrect operator
+#endif // __MINGW32__
 
 #include "rapidjson/document.h"
 #include "rapidjson/stringbuffer.h"
 #include "rapidjson/writer.h"
 
+#ifndef __MINGW32__
 #pragma warning(pop)
+#endif // __MINGW32__
 
 // if only there was a standard library function for this
 template <size_t Len>