Improve --enable-gpu
At some point the ability to target a single compute capability was lost. This restores that.
I tested these three variations and they have reasonable results (all, only compute capability 61, error)
autoheader && autoreconf -i && ./configure --enable-gpu --with-cuda=/usr/local/cuda CC=gcc-8
autoheader && autoreconf -i && ./configure --enable-gpu=61 --with-cuda=/usr/local/cuda CC=gcc-8
autoheader && autoreconf -i && ./configure --enable-gpu=89 --with-cuda=/usr/local/cuda CC=gcc-8
This also drops compute capabilities 20 and 21 but those are very old (circa 2009 GPUs) and long deprecated in cuda.