When LL_BUILD is not in the environment at autobuild configure time, important
macros such as LL_WINDOWS aren't set. That means that platform-dependent
macros such as LL_TYPEOF() aren't defined, which can produce obscure errors
like this:

indra\llcommon\llunittype.h(51): error C2226: syntax error :
unexpected type 'S' (packages\llphysicsextensions\stub\LLPhysicsExtensionsStubImpl.cpp)
10>          indra\llcommon\llunittype.h(52) :
see reference to class template instantiation 'LLResultTypeAdd<S,T>' being compiled

Make the CMake logic fail with a more readily-understood error in that case.