Updated xs/Build.PL to support Visual Studio compiler suite.

Updated xs/Build.PL to understand BOOST_LIBRARY_PATH and
BOOST_INCLUDE_PATH environment variables. This way one may easily
switch between various boost builds.

Some minor tweeks were done to make Slic3r compile with
Visual Studio 2013.

xs/src/libslic3r/PlaceholderParser.cpp

@@ -3,14 +3,22 @@
 #include <ctime>
 #include <iomanip>
 #include <sstream>
-#include <unistd.h>  // provides **environ
+#ifdef _MSC_VER
+    #include <stdlib.h>  // provides **_environ
+#else
+    #include <unistd.h>  // provides **environ
+#endif
 
 #ifdef __APPLE__
 #include <crt_externs.h>
 #undef environ
 #define environ (*_NSGetEnviron())
 #else
- 	extern char **environ;
+    #ifdef _MSC_VER
+       #define environ _environ
+    #else
+     	extern char **environ;
+    #endif
 #endif
 
 namespace Slic3r {