Macroprocessor: fix line counter inside @{} constructs

There were two bugs:
– with the LF convention, newlines were counted twice
– with the CR+LF convention, they were counted four times (because the CR was
  included in yyleng, alongside the LF)

The fix consists in implementing a location_increment() method similar to the
one used for the Dynare parser. This is the most robust solution.

By the way, mark the method DynareFlex::location_increment() method static.