Various adjustments to make the config.h output between autotools
and meson more similar by testing on Linux and Windows/MSYS2.
Setting things to 1 instead of true and shifting things around is motivated
by reducing the diff between the generated files.
This changes the configure option into two states:
auto: build all that can be build (default)
A list of backend names: build them and fail if we can't
"papi" is missing because it's not in Debian and I can't test it.