Allow disabling of gdbserver build (--enable-gdbserver=yes/no/auto).

This patch adds a new --enable-gdbserver=yes/no/auto command-line switch
in gdb/configure.  The primary purpose is to allow a user to disable the
build & install of gdbserver when not desired.  It also allows the user
to request gdbserver in which case the configure script will abort if
automatic building of gdbserver is not supported for that configuration.

The default keeps things as is: We automatically build gdbserver if
building for a native configuration and if gdbserver is supported for
that configuration.

gdb/ChangeLog:

        * configure.ac: Add support for --enable-gdbserver.
        * configure: Regenerate.

At AdaCore, this is mostly intended for ia64-linux, where we currently
do not want to distribute gdbserver.  This will be useful in the context
of I417-033, where we are now using "make install" to determine what
needs to be packaged inside the GNAT Pro package.

gdb/configure

@@ -975,6 +975,7 @@ with_tk
 with_x
 enable_sim
 enable_multi_ice
+enable_gdbserver
 '
       ac_precious_vars='build_alias
 host_alias
@@ -1635,6 +1636,8 @@ Optional Features:
                           gcc is used
   --enable-sim            link gdb with simulator
   --enable-multi-ice      build the multi-ice-gdb-server
+  --enable-gdbserver      automatically build gdbserver (yes/no/auto, default
+                          is auto)
 
 Optional Packages:
   --with-PACKAGE[=ARG]    use PACKAGE [ARG=yes]
@@ -15816,8 +15819,20 @@ if test "x$enable_multi_ice" = xyes; then
 
 fi
 
-# We only build gdbserver automatically in a native configuration.
-if test "$gdb_native" = "yes"; then
+# Check whether --enable-gdbserver was given.
+if test "${enable_gdbserver+set}" = set; then :
+  enableval=$enable_gdbserver; case "${enableval}" in
+  yes| no|auto) ;;
+  *) as_fn_error "bad value ${enableval} for --enable-gdbserver option" "$LINENO" 5 ;;
+esac
+else
+  enable_gdbserver=auto
+fi
+
+
+# We only build gdbserver automatically in a native configuration, and
+# only if the user did not explicitly disable its build.
+if test "$gdb_native" = "yes" -a "$enable_gdbserver" != "no"; then
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether gdbserver is supported on this host" >&5
 $as_echo_n "checking whether gdbserver is supported on this host... " >&6; }
   if test "x$build_gdbserver" = xyes; then
@@ -15825,12 +15840,19 @@ $as_echo_n "checking whether gdbserver is supported on this host... " >&6; }
 $as_echo "yes" >&6; }
     subdirs="$subdirs gdbserver"
 
+    gdbserver_build_enabled=yes
   else
     { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
   fi
 fi
 
+# If the user explicitly request the gdbserver to be built, verify that
+# we were in fact able to enable it.
+if test "$enable_gdbserver" = "yes" -a "$gdbserver_build_enabled" != "yes"; then
+  as_fn_error "Automatic gdbserver build is not supported for this configuration" "$LINENO" 5
+fi
+
 # If nativefile (NAT_FILE) is not set in config/*/*.m[ht] files, we link
 # to an empty version.