I recall that @jpakkane never wanted this, but @nirbheek did, but then
@nirbheek changed his mind.
I am fine either way except for the cross inconsistency that exists
today: There is no `c_preproc_args` or similar one can put in the cross
file, so no way to replicate the effect of CPPFLAGS during cross
compilation.
This adds a hidden option to dump the current otherwise hidden peristant
state in coredata.dat.
This interface is unstable as meson has no compatibility promises about
coredata.dat.