Optimized pointer arithmetics generated with '-C, --contexts'.

Optimized pointer arithmetics generated with '-C, --contexts'.

If default input API is used and fixed-length context is based on
the rightmost context (that is, YYCURSOR), then the following expression
(that calculates pointer corresponding to the given context):
    (YYCTXMARKER + ((YYCURSOR - YYCTXMARKER) - yyctx))
can be optimized to:
    (YYCURSOR - yyctx)

Note: unfortunately, as of GCC < 7.0, expressions like:
    (YYCURSOR - 3) - (YYCURSOR - 5)
trigger warning:
    warning: integer overflow in expression [-Woverflow]
See this GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61240