Most feature checks already use #ifdef, and do not care about
the value of the macro. Convert all feature checks to do that,
and simplify the meson build scripts by replacing
if cond
cdata.set('X', 1)
endif
with
cdata.set('X', cond)
Mark some structures, arrays static/const at various places.
In some cases this prevents unnecessary initialization
when a function is entered.
All in all, the text segments across all shared
libraries are reduced by about 2 KiB. However,
the total size increases by about 2 KiB as well.
