gdb_if: use #defines for max_port

The MSVC compiler is extremely picky when it comes to assigning const
values. Whereas GCC will allow you to add two `const` variables, MSVC
will not let you use any input variables when assigning to a global
const.

Work around this by using the C preprocessor to define `MAX_PORT` that
includes a CPP definition of `DEFAULT_PORT`, then combine these two to
assign to `max_port`.

This fixes the build under MSVC.

Signed-off-by: Sean Cross <[email protected]>

src/platforms/hosted/gdb_if.c

@@ -64,8 +64,9 @@ typedef int32_t socket_t;
 #include "bmp_hosted.h"
 #include "command.h"
 
-static const uint16_t default_port = 2000U;
-static const uint16_t max_port = default_port + 4U;
+#define DEFAULT_PORT 2000U
+static const uint16_t default_port = DEFAULT_PORT;
+static const uint16_t max_port = (DEFAULT_PORT + 4U);
 
 #if defined(_WIN32) || defined(__CYGWIN__)
 const int op_would_block = WSAEWOULDBLOCK;