"-flto -O2" shouln't opt out "undefined reference" error

[Xuefer]

without -flto or without -O2 produce good (expected) result:
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configure:5332: checking for dlsym
configure:5369: clang -o conftest -O2 -pipe -march=ivybridge -O2 -pipe
  conftest.c  >&5
/tmp/conftest-597c62.o:conftest.c:function main: error: undefined
reference to 'dlsym'
clang-3.9: error: linker command failed with exit code 1 (use -v to
see invocation)
configure:5372: $? = 1
configure: failed program was:
#line 5338 "configure"
#include "confdefs.h"
/* System header to define __stub macros and hopefully few prototypes,
    which can conflict with char dlsym (); below.  */
#include <assert.h>
/* Override any gcc2 internal prototype to avoid an error.  */
#ifdef __cplusplus
extern "C"
#endif
/* We use char because int might match the return type of a gcc2
   builtin and then its argument prototype would still apply.  */
char dlsym ();
char (*f) ();

int
main ()
{
/* The GNU C library defines this for functions which it implements
    to always fail with ENOSYS.  Some functions are actually named
    something starting with __ and the normal name is an alias.  */
#if defined (__stub_dlsym) || defined (__stub___dlsym)
choke me
#else
f = dlsym; /* workaround for ICC 12.0.3 */ if (f == 0) return 1;
#endif

  ;
  return 0;
}
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 with both -flto and -O2 produce bad result (tested with clang and
gcc), this is reproduced when i build ncurses which have strange
configure script built with many workaround, unlike other configure
script

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configure:5332: checking for dlsym
configure:5369: clang -o conftest -flto -O2 -pipe -march=ivybridge -O2
-pipe   conftest.c  >&5
configure:5372: $? = 0
configure:5375: test -s conftest
configure:5378: $? = 0
configure:5388: result: yes
================================