meson: use booleans for conf.set and drop unecessary conditionals

meson: use booleans for conf.set and drop unecessary conditionals

Using conf.set() with a boolean argument does the right thing:
either #ifdef or #undef. This means that conf.set can be used unconditionally.

Previously I used '1' as the placeholder value, and that needs to be changed to
'true' for consistency (under meson 1 cannot be used in boolean context). All
checks need to be adjusted.