* configure.in: define USE_BUILTIN_FRAME_ADDRESS on x86_64
	  even if __builtin_frame_address is not used now.
	  By this, FreeBSD's clang can build ruby 1.8,
	  and alloca(1) doesn't work on it.


git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/branches/ruby_1_8_7@44719 b2dd03c8-39d4-4d8f-98ff-823fe69b080e