Revert svn_hash_gets() and svn_hash_sets() to use the original simple
implementation.  The optimizations are not worth it.

Thread: http://mid.gmane.org/CA+t0gk2hjP0hks1WS23DybKtRyHw0-aupP3wU81ptnu=o7-+Nw@mail.gmail.com

* subversion/include/svn_hash.h
  (svn_hash_gets, svn_hash_sets): Revert to 1.8 code.
    This changes the definition without changing the semantics.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509166 13f79535-47bb-0310-9956-ffa450edef68
Remove detection of __builtin_constant_p from the build system.  It is unused
as of r1509166.

* build/ac-macros/svn-macros.m4
  (SVN_CHECK_FOR_ATOMIC_BUILTINS): Remove.

* configure.ac
  (SVN_HAS_DUNDER_BUILTINS): Remove.

* subversion/svn_private_config.hw
  (SVN_HAS_DUNDER_BUILTINS): Remove.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509168 13f79535-47bb-0310-9956-ffa450edef68