New --enable-threading configure option to control use of threads in GDB/GDBserver

Add the --enable-threading configure option so multithreading can be disabled
at configure time. This is useful for statically-linked builds of
GDB/GDBserver, since the thread library doesn't play well with that setup.

If you try to run a statically-linked GDB built with threading, it will crash
when setting up the number of worker threads.

This new option is also convenient when debugging GDB in a system with lots of
threads, where the thread discovery code in GDB will emit too many messages,
like so:

[New Thread 0xfffff74d3a50 (LWP 2625599)]

If you have X threads, that message will be repeated X times.

The default for --enable-threading is "yes".

gdbserver/configure

@@ -746,6 +746,7 @@ ac_user_opts='
 enable_option_checking
 enable_maintainer_mode
 enable_largefile
+enable_threading
 with_intel_pt
 with_gnu_ld
 enable_rpath
@@ -1396,6 +1397,8 @@ Optional Features:
                           enable make rules and dependencies not useful (and
                           sometimes confusing) to the casual installer
   --disable-largefile     omit support for large files
+  --enable-threading      include support for parallel processing of data
+                          (yes/no)
   --disable-rpath         do not hardcode runtime library paths
   --enable-unit-tests     Enable the inclusion of unit tests when compiling
                           GDB
@@ -7264,6 +7267,22 @@ fi
 done
 
 
+  # ----------------------- #
+  # Check for threading.    #
+  # ----------------------- #
+
+  # Check whether --enable-threading was given.
+if test "${enable_threading+set}" = set; then :
+  enableval=$enable_threading; case "$enableval" in
+    yes) want_threading=yes ;;
+    no) want_threading=no ;;
+    *) as_fn_error $? "bad value $enableval for threading" "$LINENO" 5 ;;
+    esac
+else
+  want_threading=yes
+fi
+
+
   # Check for std::thread.  This does not work on some platforms, like
   # mingw and DJGPP.
   ac_ext=cpp
@@ -7961,10 +7980,13 @@ done
     LIBS="$save_LIBS"
     CXXFLAGS="$save_CXXFLAGS"
   fi
-  if test "$gdb_cv_cxx_std_thread" = "yes"; then
+
+  if test "$want_threading" = "yes"; then
+    if test "$gdb_cv_cxx_std_thread" = "yes"; then
 
 $as_echo "#define CXX_STD_THREAD 1" >>confdefs.h
 
+    fi
   fi
   ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'