parsing cmake variables values of bool

[zerothi]

Using fypp to parse cmake options are really great, but I generally have a problem parsing WITH_* arguments.

I would rather not do:

#:if defined("WITH_OPT")
.. option ON
#:endif

but rather something like:

#:if bool("WITH_OPT")
... option ON
#:endif

I can of course define a function that parses this, but I think this could be very generally useful? The truth/false values could optionally be set by some global FYPP_BOOL_TRUE/FALSE variables?