Add -no-undefine link flag

It is needed to build DLL with libtool. I tested it by cross-compiling
on Debian GNU/Linux sid with x86_64-w64-mingw32-gcc.

GCC version:

    % x86_64-w64-mingw32-gcc --version
    x86_64-w64-mingw32-gcc (GCC) 4.9.1
    Copyright (C) 2014 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Merge pull request #45 from kou/add-no-undefined-link-flag

Add -no-undefine link flag