c: port YY_ATTRIBUTE_UNUSED to Sun C 5.12

Sun C 5.12 defines __SUNPRO_C to 0x5120 but diagnoses
‘__attribute__ ((__unused__))’.  Change the ifdefs to use
the same method as Gnulib in this area.
* data/skeletons/c.m4 (YY_ATTRIBUTE): Remove, since
not all attributes were added in the same compiler version.
(YY_ATTRIBUTE_PURE, YY_ATTRIBUTE_UNUSED):
Use specific GCC version for each attribute.
Pay no attention to __SUNPRO_C.
* tests/headers.at (Several parsers): Tighten tests accordingly.

data/skeletons/c.m4

@@ -282,22 +282,20 @@ m4_define([b4_table_value_equals],
 # Provide portable compiler "attributes".  If "noreturn" is passed, define
 # _Noreturn.
 m4_define([b4_attribute_define],
-[[#ifndef YY_ATTRIBUTE
+[[#ifndef YY_ATTRIBUTE_PURE
-# if (defined __GNUC__                                               \
+# if defined __GNUC__ && 2 < __GNUC__ + (96 <= __GNUC_MINOR__)
-      && (2 < __GNUC__ || (__GNUC__ == 2 && 96 <= __GNUC_MINOR__)))  \
+#  define YY_ATTRIBUTE_PURE __attribute__ ((__pure__))
-     || defined __SUNPRO_C && 0x5110 <= __SUNPRO_C
-#  define YY_ATTRIBUTE(Spec) __attribute__(Spec)
 # else
-#  define YY_ATTRIBUTE(Spec) /* empty */
+#  define YY_ATTRIBUTE_PURE
 # endif
 #endif
 
-#ifndef YY_ATTRIBUTE_PURE
-# define YY_ATTRIBUTE_PURE   YY_ATTRIBUTE ((__pure__))
-#endif
-
 #ifndef YY_ATTRIBUTE_UNUSED
-# define YY_ATTRIBUTE_UNUSED YY_ATTRIBUTE ((__unused__))
+# if defined __GNUC__ && 2 < __GNUC__ + (7 <= __GNUC_MINOR__)
+#  define YY_ATTRIBUTE_UNUSED __attribute__ ((__unused__))
+# else
+#  define YY_ATTRIBUTE_UNUSED
+# endif
 #endif
 
 ]m4_bmatch([$1], [\bnoreturn\b], [[/* The _Noreturn keyword of C11.  */

tests/headers.at

@@ -321,7 +321,7 @@ AT_PERL_CHECK([[-n -0777 -e '
       |YYChar
       |YYPUSH_MORE(?:_DEFINED)?
       |YYUSE
-      |YY_ATTRIBUTE(?:_PURE|_UNUSED)?
+      |YY_ATTRIBUTE(?:_PURE|_UNUSED)
       |YY_CONSTEXPR
       |YY_COPY
       |YY_CPLUSPLUS