lexer: fix lpeg-0.12 empty string in loop error.

* macro/lexer.lua (lexer.init): Replace longstring callback with
the longstring matcher from the lpeg 0.12 documentation.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
Merge pull request #4 from gvvaughan/pull-request/fix-lpeg12-empty-string-loop

lexer: fix lpeg-0.12 empty string in loop error.