Change default compiler to ‘g++’

‘amded’ defaulted to using ‘clang++’ since the beginning of the C++
rewrite of the project. However, due to the ridiculous build times that
are happening with clang as of late, I'm switching the default CXX
definition to "g++".

What do I mean by ridiculous build time? Well, observe:

  % time make CXX=clang++ > /dev/null
  make CXX=clang++ > /dev/null  127.66s user 1.54s system 93% cpu 2:18.44 total

  % time make CXX=g++ > /dev/null
  make CXX=g++ > /dev/null  17.09s user 1.50s system 94% cpu 19.688 total

This is unacceptable.

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>