build: add an option to control WMO

WMO requires the new swift-driver which requires an early swift-driver
or a host toolchain that has Swift.  Since the only platform that
guarantees this currently is Darwin, use a conditional value for the
option, defaulting to `NO` for most other targets.  This should repair
the build on Windows.

Author: compnerd

CMakeLists.txt

@@ -23,6 +23,10 @@ set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib/${SWIFT_HOST_LIBRARI
 
 set(CMAKE_MACOSX_RPATH YES)
 
+option(SWIFT_DRIVER_ENABLE_WMO
+       "Enable Whole Module Optimization (WMO) - requires swift-driver"
+       $<IF:$<PLATFORM_ID:Darwin>,YES,NO>)
+
 include(AddSwiftHostLibrary)
 
 # Ensure that we do not link the _StringProcessing module. But we can

cmake/modules/AddSwiftHostLibrary.cmake

@@ -56,7 +56,7 @@ function(add_swift_host_library name)
       -emit-module-interface-path;${module_interface_file}
       >)
 
-  if(NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
+  if(SWIFT_DRIVER_ENABLE_WMO)
     target_compile_options(${name} PRIVATE
         $<$<COMPILE_LANGUAGE:Swift>:
         -wmo