libstdc++: Define __cpp_lib_to_chars for C++20 [PR 100146]
This defines the feature test macro when uselocale is available, because
the floating-point std::from_chars support currently depends on that.
Co-authored-by: Jakub Jelinek <jakub@redhat.com>
libstdc++-v3/ChangeLog:
PR libstdc++/100146
* include/std/charconv (__cpp_lib_to_chars): Define
conditionally.
* include/std/version (__cpp_lib_to_chars): Likewise..
* testsuite/20_util/from_chars/4.cc: Only check feature test
macro, not _GLIBCXX_HAVE_USELOCALE.
* testsuite/20_util/from_chars/5.cc: Likewise.
* testsuite/20_util/from_chars/6.cc: Likewise.
* testsuite/20_util/to_chars/long_double.cc: Likewise.