configure.ac: Create an UNUSED #define for parameter passing

Some functions in libdvdread take parameters that are unused, e.g
file_read() in dvd_input.c. These functions can't be changed to remove
the unused parameters as they are public API.

GCC provides a __attribute__((unused)) annotation that can be used on
function parameters to inform the compiler of such and will squash
warnings of the following nature

    src/dvd_input.c: In function 'file_read':
    src/dvd_input.c:218:69: warning: unused parameter 'flags'
      [-Wunused-parameter]
     static int file_read(dvd_input_t dev, void *buffer, int blocks,
                          int flags)
                              ^

A check is added to configure.ac for the use of GCC and if found it
creates an UNUSED #define set to __attribute__((unused)) otherwise it
just sets it empty.

If compilers on other systems have a similar feature this check can be
adjusted to accommodate them.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
src/dvd_input.c: Annotate unused function parameters

In libdvdread some functions take parameters that are unused. These
functions are for working without the libdvdcss library but have to have
the same prototype as the ones for working with it as these are
presented as single API.

As such we get the following warnings from GCC (4.8.3)

    src/dvd_input.c: In function 'file_error':
    src/dvd_input.c:186:37: warning: unused parameter 'dev'
      [-Wunused-parameter]
     static char *file_error(dvd_input_t dev)
                                         ^
    src/dvd_input.c: In function 'file_title':
    src/dvd_input.c:210:35: warning: unused parameter 'dev'
      [-Wunused-parameter]
     static int file_title(dvd_input_t dev, int block)
                                       ^
    src/dvd_input.c:210:44: warning: unused parameter 'block'
      [-Wunused-parameter]
     static int file_title(dvd_input_t dev, int block)
                                                ^
    src/dvd_input.c: In function 'file_read':
    src/dvd_input.c:218:69: warning: unused parameter 'flags'
      [-Wunused-parameter]
     static int file_read(dvd_input_t dev, void *buffer, int blocks,
                          int flags)
                              ^
The previous commit added a #define set to __attribute__((unused)) when
GCC is being used (empty otherwise). This can be used to inform the
compiler about such unused parameters and gets rid of those warnings.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>