ttyplot: ttyplot doesn't build (in general and on Void Linux) because of issues in the Makefile

I’m updating the Void Linux package and found several problems with the Makefile for which I provide a PR. Back quotes are not used for shell expansion in Make because a Makefile is not a shell script. The proper way is through $(shell pkg-config ...). Also if CFLAGS are passed through make CFLAGS=.... (like Void Linux does) the += statements in Makefile have no effect. So I added override where it’s needed.