regex: improve Gnulib port to AIX

From the glibc point of view, this removes duplicate macro
definitions and is obviously safe.
From the Gnulib point of view, this pacifies xlc 12.01 on AIX 7.1.
* posix/regex_internal.h:
(__attribute__, __attribute_warn_unused_result__):
Remove; already defined elsewhere.

ChangeLog

@@ -1,3 +1,13 @@
+2018-12-31  Paul Eggert  <[email protected]>
+
+	regex: improve Gnulib port to AIX
+	From the glibc point of view, this removes duplicate macro
+	definitions and is obviously safe.
+	From the Gnulib point of view, this pacifies xlc 12.01 on AIX 7.1.
+	* posix/regex_internal.h:
+	(__attribute__, __attribute_warn_unused_result__):
+	Remove; already defined elsewhere.
+
 2018-12-31  Florian Weimer  <[email protected]>
 
 	[BZ #24027]

posix/regex_internal.h

@@ -146,10 +146,6 @@
 # define __regfree regfree
 #endif /* not _LIBC */
 
-#if !__GNUC_PREREQ (3, 1)
-# define __attribute__(arg)
-#endif
-
 #ifndef SSIZE_MAX
 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
 #endif
@@ -867,14 +863,6 @@ re_string_elem_size_at (const re_string_t *pstr, Idx idx)
 }
 #endif /* RE_ENABLE_I18N */
 
-#if __GNUC_PREREQ (3,4)
-# undef __attribute_warn_unused_result__
-# define __attribute_warn_unused_result__ \
-   __attribute__ ((__warn_unused_result__))
-#else
-# define __attribute_warn_unused_result__ /* empty */
-#endif
-
 #ifndef FALLTHROUGH
 # if __GNUC__ < 7
 #  define FALLTHROUGH ((void) 0)