libstdc++: Allow 'configure.host' to pre-set 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'

In particular, 'GLIBCXX_ENABLE_CXX_FLAGS' shouldn't overwrite 'EXTRA_CXX_FLAGS'
(and instead append any additional '--enable-cxx-flags=[...]').

	libstdc++-v3/
	* acinclude.m4 (GLIBCXX_ENABLE_CXX_FLAGS): Append to
	'EXTRA_CXX_FLAGS' any additional flags.
	* configure: Regenerate.
	* configure.host: Document 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'.

libstdc++-v3/acinclude.m4

@@ -3267,9 +3267,11 @@ AC_DEFUN([GLIBCXX_ENABLE_CXX_FLAGS], [dnl
 	     AC_MSG_ERROR([compiler flags start with a -]) ;;
       esac
     done
+
+    # Append the additional flags to any that came from 'configure.host'.
+    EXTRA_CXX_FLAGS="$EXTRA_CXX_FLAGS $enable_cxx_flags"
   fi
 
-  EXTRA_CXX_FLAGS="$enable_cxx_flags"
   AC_MSG_RESULT($EXTRA_CXX_FLAGS)
   AC_SUBST(EXTRA_CXX_FLAGS)
 ])

libstdc++-v3/configure

@@ -19450,9 +19450,11 @@ fi
 	     as_fn_error $? "compiler flags start with a -" "$LINENO" 5 ;;
       esac
     done
+
+    # Append the additional flags to any that came from 'configure.host'.
+    EXTRA_CXX_FLAGS="$EXTRA_CXX_FLAGS $enable_cxx_flags"
   fi
 
-  EXTRA_CXX_FLAGS="$enable_cxx_flags"
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: $EXTRA_CXX_FLAGS" >&5
 $as_echo "$EXTRA_CXX_FLAGS" >&6; }
 

libstdc++-v3/configure.host

@@ -61,6 +61,10 @@
 #
 # It possibly modifies the following variables:
 #
+#   EXTRA_CFLAGS           extra flags to pass when compiling C code
+#
+#   EXTRA_CXX_FLAGS        extra flags to pass when compiling C++ code
+#
 #   OPT_LDFLAGS            extra flags to pass when linking the library, of
 #                          the form '-Wl,blah'
 #                          (defaults to empty in acinclude.m4)