Abstracted XML parser library and switched to autotools building.

expat is still the default parser, but libxml2 can also be used
via an option to the configure script.  New parsers can easily be
added by implementing a parser_foo.c that uses the interface defined
in parser.h.

.gitignore

@@ -14,6 +14,7 @@ autom4te.cache
 src/*.o
 examples/*.o
 tests/*.o
+tests/check_parser
 *.a
 examples/basic
 examples/active