use extended regular expressions in the test for std::regex

for some compilers (e.g., GCC < 4.9) the default is buggy and
opm-parser thus uses extended expressions...

thanks to [at]bska for digging this up!

this is the leftover of #259 after #260 was merged. Although this likely won't break the other modules, I'll do another build system synchronization after this has been merged...