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__.