CMake build process - fixes of the previous check-ins to build

cleanly on Windows & Linux. There is still a work on OSX:
The XS module has to be linked without perl.lib and with
the following parameters to produce a bundle: -bundle -undefined dynamic_lookup

xs/src/libslic3r/GCodeSender.hpp

@@ -1,6 +1,5 @@
 #ifndef slic3r_GCodeSender_hpp_
 #define slic3r_GCodeSender_hpp_
-#ifdef BOOST_LIBS
 
 #include "libslic3r.h"
 #include <queue>
@@ -71,5 +70,4 @@ class GCodeSender : private boost::noncopyable {
 
 } // namespace Slic3r
 
-#endif /* BOOST_LIBS */
 #endif /* slic3r_GCodeSender_hpp_ */