Fix handling of strings in wxPrintf() when using recent MinGW

MinGW-w64 (and apparently TDM too) defaults to using ANSI stdio functions
nowadays, which interpret format specifiers such as "%s" and "%c" in wide
string functions in the standard-conforming way, i.e. still expecting the
arguments of "char*" type, and not "wchar_t*" as MSVC and older MinGW did.

docs/changes.txt

@@ -207,6 +207,7 @@ wxMSW:
 - Return correct OS version under Windows 8.1 and later.
 - Fix crash in wxD2DContext when using non-MSVC compiler (iwbnwif).
 - Notify shell about the changes done by wxMimeTypesManager (Maarten Bent).
+- Fix wxPrintf() and friends when using MinGW with ANSI stdio option.
 
 wxOSX/Cocoa:
 

src/common/strvararg.cpp

@@ -410,7 +410,13 @@ private:
     size_t m_nCopied;
 };
 
-#if defined(__WINDOWS__) && !defined(__CYGWIN__)
+// Distinguish between the traditional Windows (and MSVC) behaviour and Cygwin
+// (which is always Unix-like) and MinGW which can use either depending on the
+// "ANSI stdio" option value (which is normally set to either 0 or 1, but test
+// for it in a way which works even if it's not defined at all, just in case).
+#if defined(__WINDOWS__) && \
+    !defined(__CYGWIN__) && \
+    (!defined(__MINGW32__) || (__USE_MINGW_ANSI_STDIO +0 == 0))
 
 // on Windows, we should use %s and %c regardless of the build:
 class wxPrintfFormatConverterWchar : public wxFormatConverterBase<wchar_t>