Stop 'configure --enable-threading' if std::thread doesn't work

Currently, if you configure gdb with explicit --enable-threading, but
then configure detects std::thread does not work, configure silently
disables threading support and continues configuring.

This patch makes that scenario cause a configuration error, like so:

 $ /home/pedro/gdb/src/configure --enable-threading && make
 ...
 configure: error: std::thread does not work; disable threading
 make[1]: *** [Makefile:11225: configure-gdbsupport] Error 1
 make[1]: Leaving directory '/home/pedro/gdb/build-windows-threads'
 make: *** [Makefile:1041: all] Error 2
 $

Additionally, if you don't explicitly pass --enable-threading, and
std::thread does not work, we will now get a warning (and the build
continues):

 $ /home/pedro/gdb/src/configure && make
 ...
 configure: WARNING: std::thread does not work; disabling threading
 ...

This is similar to how we handle --enable-tui and missing curses.  The
code and error/warning messages were borrowed from there.

Change-Id: I73a8b580d1e2a796b23136920c0e181408ae1b22
Approved-By: Tom Tromey <tom@tromey.com>

gdb/configure

@@ -20181,12 +20181,13 @@ if test "${enable_threading+set}" = set; then :
     *) as_fn_error $? "bad value $enableval for threading" "$LINENO" 5 ;;
     esac
 else
-  want_threading=yes
+  want_threading=auto
 fi
 
 
   # Check for std::thread.  This does not work on some platforms, like
-  # mingw and DJGPP.
+  # mingw using the win32 threads model with gcc older than 13, and
+  # DJGPP.
   ac_ext=cpp
 ac_cpp='$CXXCPP $CPPFLAGS'
 ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
@@ -20893,11 +20894,18 @@ done
   LIBS="$save_LIBS"
   CXXFLAGS="$save_CXXFLAGS"
 
-  if test "$want_threading" = "yes"; then
+  if test "$want_threading" != "no"; then
     if test "$gdb_cv_cxx_std_thread" = "yes"; then
 
 $as_echo "#define CXX_STD_THREAD 1" >>confdefs.h
 
+    else
+	if test "$want_threading" = "yes"; then
+	    as_fn_error $? "std::thread does not work; disable threading" "$LINENO" 5
+	else
+	    { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: std::thread does not work; disabling threading" >&5
+$as_echo "$as_me: WARNING: std::thread does not work; disabling threading" >&2;}
+	fi
     fi
   fi
   ac_ext=c