fix compilation with boost::filesystem

CMakeLists.txt:
---------------------------
Until now build with boost::filesystem was completely broken,
since cmake exported definitions with wrong name prefixes, and
nana always defaulted to internal filesystem implementation.

After fixing the boost definitions, a number of errors came up
due to incompatibility of boost::filesystem with nana and std
filesystems. This commit tries to fix them all.

filesystem.cpp, filesystem.hpp, filebox.cpp:
--------------------------------------------
boost::filesystem doesn't have a file_time_type, so declared
it in the filesystem.hpp header.

boost::filesystem::last_write_time has a return type std::time_t
unlike the other two implementations of this function in nana
and std, so added ifdef to convert the result to file_time_type.

fixed build on gcc-4.9, since it doesn't have a std::put_time
function, included <nana/stdc++.hpp> in that case.

boost::filesystem::file_type types have different names than
std::experimental::filesystem::file_type types, fixed it by
creating an enum class file_type with the same type names as
in std::experimental::filesystem::file_type. This fix
requires static_cast from functions results to internal file_type,
since boost file_type and std file_type a different enum classes.

changed switch to if, bacause old gcc fails on converting
enum class members to int.

stdc++.hpp:
-----------
added ifndef guards to prevent errors on multiple includes of this header.

wvl.cpp:
--------
added boost/chrono.hpp include for the cases when std::thread is not
available.

travis:
-------
added boost system, thread, chrono libs to install, they are needed
for the nana-demo to compile.

include/nana/filesystem/filesystem.hpp

@@ -53,6 +53,7 @@
 
 #undef  NANA_USING_BOOST_FILESYSTEM
 #define NANA_USING_BOOST_FILESYSTEM 1
+#   include <chrono>
 #   include <boost/filesystem.hpp>
 
 // add boost::filesystem into std::experimental::filesystem
@@ -60,6 +61,20 @@ namespace std {
 	namespace experimental {
 		namespace filesystem {
 			using namespace boost::filesystem;
+			using file_time_type = std::chrono::time_point<std::chrono::system_clock>;
+
+			enum class file_type {
+				none = boost::filesystem::file_type::status_unknown,
+				not_found = boost::filesystem::file_type::file_not_found,
+				regular = boost::filesystem::file_type::regular_file,
+				directory = boost::filesystem::file_type::directory_file,
+				symlink = boost::filesystem::file_type::symlink_file,
+				block = boost::filesystem::file_type::block_file,
+				character = boost::filesystem::file_type::character_file,
+				fifo = boost::filesystem::file_type::fifo_file,
+				socket = boost::filesystem::file_type::socket_file,
+				unknown = boost::filesystem::file_type::type_unknown,
+			};
 		} // filesystem
 	} // experimental
 } // std

include/nana/stdc++.hpp

@@ -12,6 +12,9 @@
  *	@brief Implement the lack support of standard library.
  */
 
+#ifndef NANA_STDCXX_INCLUDED
+#define NANA_STDCXX_INCLUDED
+
 #include "c++defines.hpp"
 
 //Implement workarounds for GCC/MinGW which version is below 4.8.2
@@ -153,4 +156,6 @@ namespace std
 		return (v < lo ? lo : (hi < v ? hi : v));
 	}
 }
-#endif
+#endif
+
+#endif // NANA_STDCXX_INCLUDED