config_file.l: Change default prefix for global symbols.

Without this, applications which use flex for their own work might run
into problems due to duplicate symbols when linking against liblopsub.
Since this is an ABI change, we have to increment the ABI version
number.

No changes in the source code are necessary since flex provides macros
which allow to still refer to the variables and functions with the
usual yy prefixed names. However, the command

	nm -e liblopsub.a

shows that the external symbols are now prefixed with lls_yy.

Note that the patch only changes config_file.l. We don't bother to
adjust lopsubgen.l in the same way because the object file which
is derived from lopsubgen.l is not part of the library. It is only
linked into the lopsubgen executable.