libstdc++: Strengthen the check for availability of pthread_rwlock_t

        * acinclude.m4 (_GLIBCXX_USE_PTHREAD_RWLOCK_T): Checks that
        _PTHREADS is defined after including gthr.h.
        * configure: Regenerate.

From-SVN: r279635

libstdc++-v3/ChangeLog

+2019-12-20  Jerome Lambourg  <lambourg@adacore.com>
+
+	* acinclude.m4 (_GLIBCXX_USE_PTHREAD_RWLOCK_T): Checks that _PTHREADS
+	is defined after including gthr.h.
+	* configure: Regenerate.
+
 2019-12-12  Jonathan Wakely  <jwakely@redhat.com>
 
 	* libsupc++/compare (common_comparison_category): Define without using

libstdc++-v3/acinclude.m4

@@ -4017,12 +4017,24 @@ AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [
 	      [Define if gthreads library is available.])
 
     # Also check for pthread_rwlock_t for std::shared_timed_mutex in C++14
+    # but only do so if we're using pthread in the gthread library.
+    # On VxWorks for example, pthread_rwlock_t is defined in sys/types.h
+    # but the pthread library is not there by default and the gthread library
+    # does not use it.
+    AC_TRY_COMPILE([#include "gthr.h"],
+    [
+      #if (!defined(_PTHREADS))
+      #error
+      #endif
+    ], [ac_gthread_use_pthreads=yes], [ac_gthread_use_pthreads=no])
+    if test x"$ac_gthread_use_pthreads" = x"yes"; then
       AC_CHECK_TYPE([pthread_rwlock_t],
              [AC_DEFINE([_GLIBCXX_USE_PTHREAD_RWLOCK_T], 1,
              [Define if POSIX read/write locks are available in <gthr.h>.])],
              [],
              [#include "gthr.h"])
     fi
+  fi
 
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE

libstdc++-v3/configure

@@ -74654,6 +74654,32 @@ $as_echo "#define _GLIBCXX_HAS_GTHREADS 1" >>confdefs.h
 
 
     # Also check for pthread_rwlock_t for std::shared_timed_mutex in C++14
+    # but only do so if we're using pthread in the gthread library.
+    # On VxWorks for example, pthread_rwlock_t is defined in sys/types.h
+    # but the pthread library is not there by default and the gthread library
+    # does not use it.
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include "gthr.h"
+int
+main ()
+{
+
+      #if (!defined(_PTHREADS))
+      #error
+      #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ac_gthread_use_pthreads=yes
+else
+  ac_gthread_use_pthreads=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+    if test x"$ac_gthread_use_pthreads" = x"yes"; then
       ac_fn_cxx_check_type "$LINENO" "pthread_rwlock_t" "ac_cv_type_pthread_rwlock_t" "#include \"gthr.h\"
 "
 if test "x$ac_cv_type_pthread_rwlock_t" = xyes; then :
@@ -74663,6 +74689,7 @@ $as_echo "#define _GLIBCXX_USE_PTHREAD_RWLOCK_T 1" >>confdefs.h
 fi
 
     fi
+  fi
 
   CXXFLAGS="$ac_save_CXXFLAGS"
   ac_ext=c