[flang][cmake] Set the default for FLANG_BUILD_NEW_DRIVER for oot builds

For out-of-tree builds of Flang, FLANG_BUILD_NEW_DRIVER is not inherited
from llvm-project/llvm/CMakeLists.txt. Instead, a separate definition is
required (but only for out-of-tree builds).

Differential Revision: https://reviews.llvm.org/D102323