* config/abi/pre/gnu.ver (GLIBCXX_3.4.20): Correct regex_error export. (GLIBCXX_3.4.21): Export base object constructor for regex_error. * acinclude.m4 (libtool_VERSION): Bump. * configure: Regenerate. * testsuite/util/testsuite_abi.cc: Add GLIBCXX_3.4.21 version. * testsuite/28_regex/regex_error/base.cc: New. From-SVN: r210290