Issue 2787: Sanitize usage of -DDEBUG, -DNDEBUG and assert

Issue 2787: Sanitize usage of -DDEBUG, -DNDEBUG and assert

The compiler option -DNDEBUG is no longer being used: -DNDEBUG disables
the assert function, and assert is essentially stating that the program
cannot useful continue if the assertion is not met.  -DNDEBUG is
basically an option for compiling an application to a limited amount of
ROM when aborting with a diagnostic is not preferable to crashing.

This is not the case for LilyPond.  So expensive debugging options now
are enabled with -DDEBUG instead.  There is a new configure option
--enable-checking defaulting to "off" for this now.

At the current point of time, setting --disable-optimising also has the
effect of enabling the checks: this will be retained until Patchy has
been adapted to using --enable-checking.