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/src/xsinit.h

@@ -40,7 +40,7 @@
 // #include <libslic3r.h>
 
 #ifdef SLIC3RXS
-extern "C" {
+// extern "C" {
 #include "EXTERN.h"
 #include "perl.h"
 #include "XSUB.h"
@@ -88,7 +88,7 @@ extern "C" {
 #undef Zero
 #undef Packet
 #undef _
-}
+// }
 #endif
 
 #include <ClipperUtils.hpp>