Fix build with update checker disabled

[sarnex]

You can reproduce the issue with

meson setup build -Db_pch=false -Dwerror=false -Dbuildtype=plain -Denable_update_checker=false -Dffms2=enabled -Dsystem_luajit=true -Dalsa=enabled -Dfftw3=enabled -Dopenal=disabled -Dportaudio=disabled -Dlibpulse=enabled -Dhunspell=enabled -Duchardet=enabled  --libdir lib64 --localstatedir /var/lib --prefix /usr --sysconfdir /etc

Meson defines the DEFINE to 0 but the check assumes it will not be defined if disabled.

I'm not sure how this wasn't hit before, maybe nobody disables the update checker.

[arch1t3cht]

This was a bug in the meson logic to turn a configuration_data into a set of compiler flags. I fixed this on the meson side in 0f2c324 .

[sarnex]

Thanks, I assumed the logic there was right so I didn't even look :)

Glad I don't have to fight the preprocessor anymore