bump min version of gcc to 6.1
Bump min version of gcc to 6.1, as that has proper support for C++14 and fixes bugs in OpenMP support.
gcc 4.8 is no longer supported by any current LTS distro.
Bump min version of gcc to 6.1, as that has proper support for C++14 and fixes bugs in OpenMP support.
gcc 4.8 is no longer supported by any current LTS distro.