Follow-up to r1507366: svn_hash_gets: compute the length of string literal
keys (common case) at compile-time, without multiply-evaluating dynamically-
computed keys.

Review by: philip
           breser

* build/ac-macros/svn-macros.m4
  (SVN_CHECK_FOR_DUNDER_BUILTINS): New macro.

* configure.ac:
    Call SVN_CHECK_FOR_DUNDER_BUILTINS() and set -D SVN_HAS_DUNDER_BUILTINS.

* subversion/include/svn_hash.h
  (svn_hash_gets): Use __builtin_choose_expr() and __builtin_constant_p(),
    when available.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1508170 13f79535-47bb-0310-9956-ffa450edef68