fix build when fgviewer is manually enabled

[pixelflinger]

No description provided.

[show50726]

We may also want to uncomment the lines around 541?

[pixelflinger]

We may also want to uncomment the lines around 541?

I think that one is not strictly necessary right? That one just makes it the default when DEBUG.

[poweifeng]

We may also want to uncomment the lines around 541?

I think that one is not strictly necessary right? That one just makes it the default when DEBUG.

I think it is necessary because one could run cmake outside of build.sh, in which case, we'd get an undefined option for debug builds (I think).

[show50726]

We may also want to uncomment the lines around 541?

I think that one is not strictly necessary right? That one just makes it the default when DEBUG.

I think it is necessary because one could run cmake outside of build.sh, in which case, we'd get an undefined option for debug builds (I think).

I think there would not be error if an option is undefined (however the behavior might not always be as expected).

BTW, I think if we want to make users able to build and use fgviewer, I opened another PR with all necessary changes: #8453