Following my remark about tests XFAIL-ing when running 'make check' I'd 
like to propose this to avoid this situation.

     libstdc++: [_GLIBCXX_ASSERTIONS] Activate basic 
_GLIBCXX_DEBUB_PEDANTIC checks

     Activate __glibcxx_requires_string/__glibcxx_requires_string_len in 
basic _GLIBCXX_ASSERTIONS
     mode which is then considering _GLIBCXX_DEBUG_PEDANTIC for this 
purpose.

     Thanks to this change add _GLIBCXX_ASSERTIONS to some tests that 
are otherwise XFAIL-ing on
     segmentation fault rather than on a proper __glibcxx_assert call.

     libstdc++-v3/ChangeLog:

             * include/debug/debug.h (__glibcxx_requires_string, 
__glibcxx_requires_string_len): Move
             definitions...
             * include/debug/assertions.h: ... here. Definition now 
depends on _GLIBCXX_ASSERTIONS rather
             than _GLIBCXX_DEBUG.
             * 
testsuite/21_strings/basic_string/operations/ends_with/char_neg.cc: Add 
_GLIBCXX_ASSERTIONS
             define.
             * 
testsuite/21_strings/basic_string/operations/ends_with/wchar_t_neg.cc: 
Likewise.
             * 
testsuite/21_strings/basic_string/operations/starts_with/char_neg.cc: 
Likewise.
             * 
testsuite/21_strings/basic_string/operations/starts_with/wchar_t_neg.cc: 
Likewise.

Tested under Linux x86_64.

Ok to commit ?

François