Extracted platform defines to a separate header and include it everywhere.

This should fix readdir(_r) on 32-bit systems, which was not 64-bit after
commit c758552338 because _FILE_OFFSET_BITS=64
was not defined in directory.cpp. Also, there were filesystem-related system
calls in unique_path.cpp, where the macro was not defined either. Including
the platform header in all source files is useful for possible future changes
that may require the platform-specific macros.

Closes https://github.com/boostorg/filesystem/pull/150.

src/exception.cpp

@@ -8,6 +8,8 @@
 
 //  Library home page: http://www.boost.org/libs/filesystem
 
+#include "platform_config.hpp"
+
 #include <string>
 #include <boost/filesystem/config.hpp>
 #include <boost/filesystem/path.hpp>