-qCXX_FOR_TARGET=`echo "$CXX_FOR_TARGET" | sed 's,&,\\\&,g'`
+qCXX_FOR_TARGET=`echo "$CXX_FOR_TARGET" | sed 's,[&%],\\\&,g'`
+
+# We want to defer the evaluation of `cmd`s and shell variables in
+# CXX_FOR_TARGET when recursing in the top-level Makefile, such as for
+# bootstrap. We'll enclose CXX_FOR_TARGET_FOR_RECURSIVE_MAKE in single
+# quotes, but we still have to duplicate `$'s so that shell variables
+# can be expanded by the nested make as shell variables, not as make
+# macros.
+qqCXX_FOR_TARGET=`echo "$qCXX_FOR_TARGET" | sed -e 's,[$][$],$$$$,g'`