Fix subnormal number parsing on mingw32

Patch by Blanca Elio Rosario.

src/strconv.c

@@ -3,6 +3,9 @@
 #include <stdio.h>
 #include <string.h>
 #include <math.h>
+#ifdef __MINGW32__
+#undef __NO_ISOCEXT /* ensure stdlib.h will declare prototypes for mingw own 'strtod' replacement, called '__strtod' */
+#endif
 #include "jansson_private.h"
 #include "strbuffer.h"
 
@@ -11,6 +14,10 @@
 #include <jansson_private_config.h>
 #endif
 
+#ifdef __MINGW32__
+#define strtod __strtod
+#endif
+
 #if JSON_HAVE_LOCALECONV
 #include <locale.h>