Don't echo ignored build systems.

This adds an extra need to press <CR> for no reason. For people like me who
continually close and re-open vim, it's quite annoying.
Add ninja and GNUmakefile as a defaults

ninja is quite popular since the integration with CMake. The README.md gives an
example that works - just use it.

For GNUmakefile target,  it is the default of GNU make after all, and BSD
projects tend to use it.
Update the README

Since ninja is now default, replace it with a different esoteric build system.
Also, update the list of predefined build targets.