Fix for mingw-w64 Windows build (#597)

Author: jeroen

src/libbson/src/bson/bson-macros.h

@@ -176,7 +176,7 @@
 #define bson_str_empty0(s) (!s || !s[0])
 
 
-#if defined(_WIN32)
+#if defined(_MSC_VER)
 #define BSON_FUNC __FUNCTION__
 #elif defined(__STDC_VERSION__) && __STDC_VERSION__ < 199901L
 #define BSON_FUNC __FUNCTION__