fix compile and load dynamic lib with MinGW

* The dynamic libs were not able to find the extern symbol factory (defined in testdcl.cc):

  ```
  g++ -shared  -o libsquare.dll square.o
  square.o:square.cc:(.text$_ZN5proxyC1Ev[__ZN5proxyC1Ev]+0x36): undefined reference to `factory'
  collect2.exe: error: ld returned 1 exit status
  make: *** [libsquare.dll] Error 1
  ```

* The current solution uses a _import library_ for the DLL. It resolves the
  undefined reference still avoiding static linking. See:

http://stackoverflow.com/questions/17601949/building-a-shared-library-using-gcc-on-linux-and-mingw-on-windows