Changes introduced by 72862c2abc moved the
default configuration from within `bin/named` to a central place
`bin/includes`.
The default configuration is conditioned by several compile-time macro.
While for most of them it's fine because they are defined in the global
`config.h` file included by default to all binaries (by meson), one
specific is not defined here. `HAVE_SO_REUSEPORT_LB` was defined in
`lib/isc/include/isc/netmgr.h` which is of course not included in
`bin/includes/defaultconfig.h`.
As a result, reuseport was disabled for all platform by default, even
the supported ones. This fixes the problem by checking if reuseport is
available on the platform from meson `config.h` generation directly,
which makes `HAVE_SO_REUSEPORT_LB` available everywhere.