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

[awarzynski]

Summary

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).

[rovka]

Makes sense, thanks!

flang/CMakeLists.txt

 
 # Check for a standalone build and configure as appropriate from
 # there.
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   message("Building Flang as a standalone project.")
   project(Flang)
   set(FLANG_STANDALONE_BUILD ON)
 
+  # For in-tree builds, this variable is inherited from
+  # llvm-project/llvm/CMakeLists.txt. For out-of-tree builds, we need a
+  # separate definition.
+  option(FLANG_BUILD_NEW_DRIVER "Build the flang compiler driver" ON)
+
   set(FLANG_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
   if (NOT MSVC_IDE)
     set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
       CACHE BOOL "Enable assertions")
     # Assertions follow llvm's configuration.
     mark_as_advanced(LLVM_ENABLE_ASSERTIONS)
   endif()