Standardise ./configure --help options reporting.

Standardise ./configure --help options reporting.

Always use AC_HELP_STRING for the help text for options at ./configure,
so that the output is properly aligned.

Use proper quadrigraphs for outputting the brackets.

Always use autoconf-style [default=$foo] output to state the default, both
where it was stated before and where it wasn't.

This time, include Matthias Bolte notes regarding defaults, and removing
PFX specification from phyp.