#162 Shared library not built with xenial version

[Yakov Simkin]

Sorry, If I mis-assigned this but on previous ticket noone got assigned so I am trying to figure out how to submit properly.
Setup:
I am building on ubuntu 16.04 xenial using mingw x86_64-w64-mingw32-g++ (GCC) 5.3.1 20160211
x86_64-w64-mingw32-gcc (GCC) 5.3.1 20160211

Problem:
make reports error below but no information on which symbols are undefined. How do I determine the root cause?
libtool: warning: undefined symbols not allowed in x86_64-w64-mingw32 shared libraries; building static only
libtool: link: x86_64-w64-mingw32-ar cru .libs/libpqxx.a array.o binarystring.o connection_base.o connection.o cursor.o dbtransaction.o encodings.o errorhandler.o except.o field.o largeobject.o nontransaction.o notification.o pipeline.o prepared_statement.o result.o robusttransaction.o sql_cursor.o statement_parameters.o strconv.o stream_base.o stream_from.o stream_to.o subtransaction.o tablereader.o tablestream.o tablewriter.o transaction.o transaction_base.o row.o util.o version.o
x86_64-w64-mingw32-ar: u' modifier ignored sinceD' is the default (see `U')
libtool: link: x86_64-w64-mingw32-ranlib .libs/libpqxx.a
libtool: link: ( cd ".libs" && rm -f "libpqxx.la" && ln -s "../libpqxx.la" "libpqxx.la" )