After five months of deprecation, mark this port for expiration in two
months (end of April 2019).

GCC 4.9 went end-of-life upstream in summer of 2016, so more than
two-and-a-half years ago, nothing depends on it any longer (and has
not for a while), and the default version of GCC in the ports tree
is now GCC 8, four major releases later.

While here stll create a proper USES block in the Makefile. [1]

Reported by: portlint [1]