mk: rework gcc version detection to permit versions newer than 4.x

mk: rework gcc version detection to permit versions newer than 4.x

Separately comparing major and minor versions becomes seriously clumsy
when with major version changes, convert the entire version string into
a numeric value (ie 4.6.0 becomes 460 and 5.0.0 becomes 500) and use
that for comparisons, eliminate unnecessary negations while at it.
This makes the comparisons simpler, more obvious and makes gcc 5.0
naturally recognized at least as capable as newest 4.x.

This three-digit scheme would run into trouble if gcc ever went to
two-digit version segments, but that hasn't happened in the last 10+
years so it seems like a safe assumption.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>