Only set COMPILER if it isn't set in the environment

The README instructions say to override `COMPILER` as an environment variable, but by default `make` gives precedence to variables defined in makefiles over environment variables, so the `COMPILER = g++` line in `src/Makefile.settings` means the environment variable is ignored.

This change only sets the variable if it doesn't already have a value, allowing the README instructions to work as intended.