A trick to make sure the code works when _XOPEN_SOURCE is defined, but empty

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6348 4407c894-4637-0410-b4f5-ada5f102cad1
restore missing code

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6349 4407c894-4637-0410-b4f5-ada5f102cad1
fix __pformat_efloat_decimal compile

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6350 4407c894-4637-0410-b4f5-ada5f102cad1