The __cpp_lib_constexpr_string and __cpp_lib_semaphore feature test
macros are not defined consistently in <version> and the relevant header
for the feature.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (__cpp_lib_constexpr_string):
Only define for C++17 and later.
* include/std/version (__cpp_lib_semaphore): Fix condition
to match the one in <semaphore>.
#if __cplusplus == 201703L
// Support P0426R1 changes to char_traits in C++17.
# define __cpp_lib_constexpr_string 201611L
-#else
+#elif __cplusplus > 201703L
// Also support P1032R1 in C++20 (but not P0980R1 yet).
# define __cpp_lib_constexpr_string 201811L
#endif
#if __cpp_lib_concepts
# define __cpp_lib_ranges 201911L
#endif
-#if __cpp_lib_atomic_wait
+#if __cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
# define __cpp_lib_semaphore 201907L
#endif
#define __cpp_lib_shift 201806L