Re-enable compiler warning flags

These were lost in the transition to Autotools. This brings them back.
Add preset zoom levels

Use a list of preset zoom levels in addition to the multiplier mode. The
zoom menu will snap to the preset levels (also accessed by Ctrl+= and
Ctrl+-), while the keyboard can still be used for finer control (+ and
-, without Ctrl).
Small text corrections

In the README, remove "in the source directory" since it doesn't
actually mean the 'src' directory.

Change a comment in utils.cpp to be less grammatically ambiguous.