Add a generating process of import library for Win32

* Warning
The author checked only the MinGW-w64 environment.

When using a lib file by MSVC, it is necessary to perform either
 of the following procedures.

1) Change the configuration of build.
    use /OPT:NOREF

2) Generate new import library with lib.exe. (MSVC Tools)
    cd $(gpac_config_dir)/bin/gcc
    lib /machine:x86(x64) /def:libgpac.def /out:libgpac.lib
Generate the .def file during configuration of Win32