Won't compile without at least one _ad variant

[Panjaksli]

Summary

The project won't compile (cryptic compile and linking errors) without specifying at least one ad variant in mitsuba.config.

It wouldn't be a problem by itself, but NOWHERE in the docs, it is ever mentioned, that one ad variant is mandatory. Not in "read the docs" online, and neither in the actual config itself... You only specify, that only mandatory variant is "scalar_rgb" and with regards to ad variants, those are only "recommended". Please make the instructions more explicit, because I've just wasted 20 hours of my life trying to make normal PT work on multiple platforms and compilers, just because I didn't need any differentiation when compiling.