Fixed unit tests on Windows after introduction of GMP to

boost::polygon Voronoi diagram generator by Vojtech.

Fixed Perl bindings on Windows after some "improvement" of the Windows 10
SDK headers, which fail if included from a C++ code using the
extern "C"
clause. Namely, the Windows 10 SDK include for sockets introduces C++ macros
if a "compiled with C++" symbol is provided even if included through
exetrn "C".

xs/main.xs.in

@@ -5,7 +5,7 @@
 // #include <libslic3r/GCodeSender.hpp>
 
 #ifdef __cplusplus
-extern "C" {
+/* extern "C" { */
 #endif
 #include "EXTERN.h"
 #include "perl.h"
@@ -14,7 +14,7 @@ extern "C" {
 #undef do_open
 #undef do_close
 #ifdef __cplusplus
-}
+/* } */
 #endif
 
 #ifdef _WIN32