Add some notes for usage on RHEL7

[rcurtin]

This comes from the discussion in #2939.

• I added a check to our CMake configuration to ensure that, if GCC is used, version 5.x or newer is used.
• I added some documentation for what to do on RHEL7/CentOS 7, where the default version of GCC is 4.8.

[zoq]

Looks good to me.

[conradsnicta]

* I added a check to our CMake configuration to ensure that, if GCC is used, version 5.x or newer is used.

If we're bumping the minimum GCC version, might as well jump to version 6 or even 7. No point in living in the ancient past.

GCC 6 is the first version with stable C++11/C++14 support, including the standard library components. The default compilation mode in GCC 6 is C++14. GCC 6 was released in April 2016 (ie. more than 5 years ago).

• mlpack requires at least Ubuntu 18.04, which has GCC 7: https://packages.ubuntu.com/bionic/gcc
• Debian stretch (currently oldstable) has GCC 6: https://packages.debian.org/stretch/gcc
• Debian buster (currently stable) has GCC 8: https://packages.debian.org/buster/gcc

[rcurtin]

@conradsnicta I see where you're coming from, but as far as I know mlpack successfully compiles on gcc 5.x still, so I don't really want to force a user to upgrade unless the software actually will not work. If we ever find that mlpack just plain no longer compiles on gcc 5.x, and it's also true that no reasonably-recent common distributions provide it, then I think that's good impetus to bump the minimum version, but at least personally I don't see the need to do so until that time.