fix header-only use of error codes

BOOST_ERROR_CODE_HEADER_ONLY was not working when the headers are separated
from the sources (e.g. when installing the headers to fhs-style /usr/include
or the like). to avoid this, we move the implementation to an .ipp file

Signed-off-by: Tim Blechmann <tim@klingt.org>

include/boost/system/error_code.hpp

@@ -382,7 +382,7 @@ namespace boost
       {
         return m_val == 0;
       }
-#   endif 
+#   endif
 
       // relationals:
       inline friend bool operator==( const error_code & lhs,
@@ -527,7 +527,7 @@ namespace boost
 #include <boost/config/abi_suffix.hpp> // pops abi_prefix.hpp pragmas
 
 # ifdef BOOST_ERROR_CODE_HEADER_ONLY
-#   include <boost/../libs/system/src/error_code.cpp>
+#   include <boost/system/detail/error_code.ipp>
 # endif
 
 #endif // BOOST_ERROR_CODE_HPP