Initial implementation of a CMake build system for the Slic3r XS module.

Based on https://github.com/CReimer/Slic3r/tree/makefile_pr
Big thanks to @CReimer for his huge effort.

xs/main.xs.in

@@ -0,0 +1,27 @@
+#include <cstring>
+#include <cstdlib>
+#include <ostream>
+#include <sstream>
+#include <libslic3r/GCodeSender.hpp>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+#include "EXTERN.h"
+#include "perl.h"
+#include "XSUB.h"
+#include "ppport.h"
+#undef do_open
+#undef do_close
+#ifdef __cplusplus
+}
+#endif
+
+#ifdef _WIN32
+	#undef XS_EXTERNAL
+	#define XS_EXTERNAL(name) __declspec(dllexport) XSPROTO(name)
+#endif /* MSVC */
+
+MODULE = Slic3r::XS	PACKAGE = Slic3r::XS
+
+@INCLUDE_COMMANDS@