[Dar-libdar_api] dar version 2.5.x missing includes

Hi Denis,

since dar version 2.5.1 including 2.5.3 I'm experiencing problems to include 
libdar in my project gdar. The compiler races the following error:
In file included from /usr/include/dar/storage.hpp:29:0,
                 from /usr/include/dar/real_infinint.hpp:43,
                 from /usr/include/dar/infinint.hpp:31,
                 from /usr/include/dar/compressor.hpp:31,
                 from /usr/include/dar/libdar.hpp:77,
                 from mylibdar.hpp:26,
                 from gdar.cpp:22:
/usr/include/dar/on_pool.hpp:37:45: fatal error: 
/usr/include/dar/cygwin_adapt.hpp: No such file or directory
 #include "/usr/include/dar/cygwin_adapt.hpp"

In dar version 2.5.3 cat_tools.hpp seams also to be missing.

The missing header files are part of the release but are not copied during 
installation.

I compiled dar from source like:
./configure --prefix=/usr
make
make install

The system I'm using is LinuxMint 17.3, but the problem probably occurs on 
other systems as well.

The compiler option when including libdar is: `pkg-config --cflags libdar`

Do I miss some compiler or config options or are the files just missing?

Best regards,
Tobias