Ubuntu 14.04 has miniupnpc 1.6.3
REGEX was failing because the string was empty, causing cmake to
error and not generate a Makefile.
This allows systems with older versions of miniupnpc to fall back
to the statically linked version in externals.
miniupnpc.h provides MINIUPNPC_API_VERSION since 1.7 and we require 1.7
or later, so there is no reason to have version detection code for older
versions.
