Allow to pass separate configure arguments for build, host and target.

/:
	PR bootstrap/45326
	PR bootstrap/45174
	* configure.ac: Honor initial values of $build_configargs,
	$host_configargs, $target_configargs.  Mark the precious, so
	environment settings get recorded.
	* configure: Regenerate.

configure.ac

@@ -2969,10 +2969,18 @@ if test "$silent" = yes; then
 fi
 baseargs="$baseargs --disable-option-checking"
 
+# Record and document user additions to sub configure arguments.
+AC_ARG_VAR([build_configargs],
+	   [additional configure arguments for build directories])
+AC_ARG_VAR([host_configargs],
+	   [additional configure arguments for host directories])
+AC_ARG_VAR([target_configargs],
+	   [additional configure arguments for target directories])
+
 # For the build-side libraries, we just need to pretend we're native,
 # and not use the same cache file.  Multilibs are neither needed nor
 # desired.
-build_configargs="--cache-file=../config.cache ${baseargs}"
+build_configargs="$build_configargs --cache-file=../config.cache ${baseargs}"
 
 # For host modules, accept cache file option, or specification as blank.
 case "${cache_file}" in
@@ -2986,9 +2994,9 @@ esac
 
 # Host dirs don't like to share a cache file either, horribly enough.
 # This seems to be due to autoconf 2.5x stupidity.
-host_configargs="--cache-file=./config.cache ${extra_host_args} ${baseargs}"
+host_configargs="$host_configargs --cache-file=./config.cache ${extra_host_args} ${baseargs}"
 
-target_configargs=${baseargs}
+target_configargs="$target_configargs ${baseargs}"
 
 # Passing a --with-cross-host argument lets the target libraries know
 # whether they are being built with a cross-compiler or being built