gansidecl.h: Check if compiler supports __attribute__.

gansidecl.h: Check if compiler supports __attribute__.

        * gansidecl.h: Check if compiler supports __attribute__.  Provide
        definitions for ATTRIBUTE_UNUSED and ATTRIBUTE_PRINTF using
        __attribute__ when its available.  Also provide definitions for
        ATTRIBUTE_PRINTF_1, ATTRIBUTE_PRINTF_2 and ATTRIBUTE_PRINTF_3 in
        terms of ATTRIBUTE_PRINTF.
        * genoutput.c (process_template): Use ATTRIBUTE_UNUSED in place
        of __attribute__.

From-SVN: r19019