MSVC and "_DEBUG" mode

[gvanem]

When compiling with mongoose.c in a MSVC program in _DEBUG-mode
(with -MDd or -MTd), the strdup() is normally defined to _strdup_dbg() in
Microsoft's crtdbg.h:

   #if defined(_CRT_INTERNAL_NONSTDC_NAMES) && _CRT_INTERNAL_NONSTDC_NAMES
       #define   strdup(s)          _strdup_dbg(s, _NORMAL_BLOCK, __FILE__, __LINE__)
       ...

If crtdbg.h gets included after mongoose.h, it's no problem. But vice-versa could be
since src/arch_win32.h and mongoose.h redefines it to _strdup().
This will not work for a _DEBUG program (probably crash).

So I suggest this patch:

--- a/src/arch_win32.h 2021-09-13 15:24:56
+++ b/src/arch_win32.h 2021-09-24 13:27:51
@@ -51,8 +51,10 @@
 #ifndef __cplusplus
 #define snprintf _snprintf
 #define vsnprintf _vsnprintf
+#ifndef strdup
 #define strdup(x) _strdup(x)
 #endif
+#endif