Remove code that "thinks" it is using GCC 4.8 instead of the default
version of GCC (which is GCC 6 these days).

For most practical purposes USE_GCC=4.8+ is equivalent to USE_GCC=yes,
so GCC 5 for several months (and now GCC 6) have been used already since
2017-03-07 and revision 435594 -- if GCC is used at all, which is not the
default anyway.