Allow compilation with OS X 10.5 SDK.
font: Keep track of init state and init only once.
font: Make al_init_font_addon return success value.

This change potentially (if unlikely) breaks the ABI so it should NOT be
backported to the 5.0 branch.
ttf: Make al_init_ttf_addon return true for subsequent calls.

This is the usual convention for Allegro.
Prepare summary of changes for 5.1.6.

The formatting is different from previous versions.  It's slightly
easier for me to edit like this (git log produces 4 space indentation)
and I think it looks better anyway.
Bump version to 5.1.6 (WIP).
Bump version to 5.1.7 (GIT).