try_compile: Simplify generator platform and toolset propagation
We don't need to save the CMAKE_GENERATOR_{PLATFORM,TOOLSET}
values
from the cache back into the cmake
instance. They were used only to
propagate the settings into cmake
instances for try_compile
, but we
already have their values in the cmMakefile
's variables anyway. In
fact those variables are the ones we actually give to the generators.