many small adjustments to get visibility working correctly on GCC and Clang.

Tested pretty extensively in release/debug shared/linked.
I don't have MSVC on me - so I'll have to upload without testing this.
One pending issue is that test_utf8_codecvt fails.  I'm guessing that the copy in detail needs to be tweaked.
I'll get this fixed up separately.

src/basic_xml_archive.cpp

@@ -14,35 +14,35 @@
 namespace boost {
 namespace archive {
 
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_OBJECT_ID(){
     return "object_id";
 }
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_OBJECT_REFERENCE(){
     return "object_id_reference";
 }
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_CLASS_ID(){
     return "class_id";
 }
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_CLASS_ID_REFERENCE(){
     return "class_id_reference";
 }
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_CLASS_NAME(){
     return "class_name";
 }
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_TRACKING(){
     return "tracking_level";
 }
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_VERSION(){
     return "version";
 }
-BOOST_ARCHIVE_DECL const char *
+BOOST_SYMBOL_VISIBLE const char *
 BOOST_ARCHIVE_XML_SIGNATURE(){
     return "signature";
 }