[patch] libstdc++/configure: strengthen the check for availability of pthread_rwlock_t

["Jérôme Lambourg" <lambourg@adacore.com>]

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Hello,

This patch to libstdc++ configure ensures that pthread_rwlock_t is used only
when pthread is used for gthreads implementation.

The original issue is that VxWorks comes with its native tasking layer and an
optional pthread layer built above it. As pthread is an optional feature of the
kernel it may not be available on the target board.

Now, even being optional, the headers are present in the development
environment, and are in particular partially available when including the main
vxWorks.h header, and so are detected automatically as available by libstdc++.

This patch will thus refine the check and will make configure try to use
pthread_rwlock_t only when the gthr library already relies on pthread.

This was of course tested with vxworks targets, and we also verified that this
does not impact Linux (that uses pthread) or Windows (that does not use
pthread).

Best regards,
- Jerome

2019-12-16  Jerome Lambourg  <lambourg@adacore.com>

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


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diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 016b0c583d0..8401696b5b5 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/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
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index b7242157107..3c51166fe9f 100755
--- a/libstdc++-v3/configure
+++ b/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