libstdc++47: Fix iostream bug 2478

c++ iostreams were segfaulting -- apparently _M_table isn't defined in all
cases.  Fix inspired from FreeBSD configuration files.

https://bugs.dragonflybsd.org/issues/2478
Remove legacy lex source files

The ancient lex was replaced by flex recently.  The bootstrapping issue
seems to be solved, so let's go ahead and purge lex from base now.