Prevent redefinition of ssize_t (yhirose#2319)

On Windows cpp-httplib defines ssize_t, therefore applications needing
to define ssize_t for their own needs or are using libraries that do
require a means to avoid a possible incompatible redefinition.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

Author: seragh

httplib.h

@@ -205,7 +205,10 @@
 
 #pragma comment(lib, "ws2_32.lib")
 
+#ifndef _SSIZE_T_DEFINED
 using ssize_t = __int64;
+#define _SSIZE_T_DEFINED
+#endif
 #endif // _MSC_VER
 
 #ifndef S_ISREG