libstdc++: Avoid useless dependency on read_symlink from tzdb

chrono::tzdb::current_zone uses filesystem::read_symlink, which creates
a dependency on the fs_ops.o object in libstdc++.a, which then creates
dependencies on several OS functions if --gc-sections isn't used. For
more details see PR libstdc++/104167 comment 8 and comment 11.

In the cases where that causes linker failures, we probably don't have
readlink anyway, so the filesystem::read_symlink call will always fail.
Repeat the preprocessor conditions for filesystem::read_symlink in the
body of chrono::tzdb::current_zone so that we don't create a
dependency on fs_ops.o for a function that will always fail.

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (tzdb::current_zone): Check configure macros
	for POSIX readlink before using filesystem::read_symlink.

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 0fcbf6a4824e97be9e5020283856d01ca7c5bffd..d22cea7e0709be929a0a6fdbb04f067e2be5f3a5 100644 (file)
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -1635,6 +1635,9 @@ namespace std::chrono
     // TODO cache this function's result?
 
 #ifndef _AIX
+    // Repeat the preprocessor condition used by filesystem::read_symlink,
+    // to avoid a dependency on src/c++17/fs_ops.o if it won't work anyway.
+#if defined(_GLIBCXX_HAVE_READLINK) && defined(_GLIBCXX_HAVE_SYS_STAT_H)
     error_code ec;
     // This should be a symlink to e.g. /usr/share/zoneinfo/Europe/London
     auto path = filesystem::read_symlink("/etc/localtime", ec);
@@ -1653,6 +1656,7 @@ namespace std::chrono
              return tz;
          }
       }
+#endif
     // Otherwise, look for a file naming the time zone.
     string_view files[] {
       "/etc/timezone",    // Debian derivates