use std::regex if it is available

it turns out that boost::regex does not work for the libstdc++ debug
mode. This patch should fix this for sufficiently new compilers.

Note that this requires the FindCXX11Features.cmake tests pulled in
from opm-core and an additional compiler flag...
Merge pull request #256 from andlaus/use_std__regex_if_available

use std::regex if it is available