Fixed build with Boost 1.89.0 by no longer linking boost system.

Increased minimum required version of Boost to 1.69.

Updated ax_boost_base.m4 macro

Improve documentation

Add an example to --time-scale documentation

Reformat text.

Update ChangeLog

Use canonical path of font file.

Don't install FreeSans.ttf if a custom font file is specified.

Document --enable-font-file configure option.

Made command line synopsis consistent.

Clarify that file filters are apply to the full path of the file.

Updated ChangeLog.