Relax requirements for booleans in invoice line.

Accept 'y' as well as 'yes', 't' as well as true.
Add some date format checking and make sure generated dates are the
correct user selected format, not always locale format.
722200 - configure script does not pick the correct am_cv_scanf version

Add -Wall -Werror to CFLAGS to try to force failure.
Merge branch 'maint' of https://github.com/Gnucash/gnucash into maint