diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index b3423d7957a..e668d2dba27 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/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)
 ])
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index e115ee55739..78758285f21 100755
--- a/libstdc++-v3/configure
+++ b/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; }
 
diff --git a/libstdc++-v3/configure.host b/libstdc++-v3/configure.host
index 7bc43071616..933a43f241c 100644
--- a/libstdc++-v3/configure.host
+++ b/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)