Fix the detection of __builtin_constant_p support.
We cannot use __builtin_choose_expr as explained here:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449

* build/ac-macros/svn-macros.m4
  (AC_DEFUN): simply check that __builtin_constant_p works

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1508221 13f79535-47bb-0310-9956-ffa450edef68