Improve libdir usage from within ltdl on MinGW.

Improve libdir usage from within ltdl on MinGW.

* libltdl/ltdl.c (parse_dotla_file) [__WINDOWS__]: Read the
libdir entry in the .la file, but accept it only if it is not
a unix-style absolute name.
* tests/ltdl-libdir.at: New test that checks if ltdl finds an
installed module via the libdir variable in the .la file.  On
MinGW, use a Windows-style libdir name including drive spec.
* Makefile.am: Add above.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>