[gcc r15-7860] Fix 'libstdc++-v3/src/c++20/tzdb.cc' build for '__GTHREADS && !__GTHREADS_CXX0X' configurations

[Thomas Schwinge]

https://gcc.gnu.org/g:780932620d859fa9e0190edd0c408d0bedf9179e

commit r15-7860-g780932620d859fa9e0190edd0c408d0bedf9179e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 20 14:08:11 2025 +0000

    Fix 'libstdc++-v3/src/c++20/tzdb.cc' build for '__GTHREADS && !__GTHREADS_CXX0X' configurations
    
            libstdc++-v3/
            * src/c++20/tzdb.cc [__GTHREADS && !__GTHREADS_CXX0X]: Use
            '__gnu_cxx::__mutex'.
    
    Co-authored-by: Thomas Schwinge <tschwinge@baylibre.com>

Diff:
---
 libstdc++-v3/src/c++20/tzdb.cc | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 9cf9eeccc599..1a1130f9b7bd 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -35,6 +35,9 @@
 #include <atomic>     // atomic<T*>, atomic<int>
 #include <memory>     // atomic<shared_ptr<T>>
 #include <mutex>      // mutex
+#if defined __GTHREADS && ! defined _GLIBCXX_HAS_GTHREADS
+# include <ext/concurrence.h> // __gnu_cxx::__mutex
+#endif
 #include <filesystem> // filesystem::read_symlink
 
 #ifndef _AIX
@@ -97,11 +100,18 @@ namespace std::chrono
 {
   namespace
   {
-#if ! USE_ATOMIC_SHARED_PTR
 #ifndef __GTHREADS
     // Dummy no-op mutex type for single-threaded targets.
     struct mutex { void lock() { } void unlock() { } };
+#elif ! defined _GLIBCXX_HAS_GTHREADS
+    // Use __gnu_cxx::__mutex if std::mutex isn't available.
+    using mutex = __gnu_cxx::__mutex;
+# if ! USE_ATOMIC_SHARED_PTR && defined __GTHREAD_MUTEX_INIT
+#  error "TODO: __gnu_cxx::__mutex can't be initialized with 'constinit'"
+# endif
 #endif
+
+#if ! USE_ATOMIC_SHARED_PTR
     inline mutex& list_mutex()
     {
 #ifdef __GTHREAD_MUTEX_INIT