Fix CMake build on macOS/BSDs

Author: owenv

Sources/Subprocess/CMakeLists.txt

@@ -40,10 +40,14 @@ elseif(APPLE)
     Platforms/Subprocess+BSD.swift
     Platforms/Subprocess+Darwin.swift
     Platforms/Subprocess+Unix.swift)
+  target_compile_options(Subprocess PRIVATE
+    "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-D SUBPROCESS_ASYNCIO_DISPATCH>")
 elseif(FREEBSD OR OPENBSD)
   target_sources(Subprocess PRIVATE
     Platforms/Subprocess+BSD.swift
     Platforms/Subprocess+Unix.swift)
+  target_compile_options(Subprocess PRIVATE
+    "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-D SUBPROCESS_ASYNCIO_DISPATCH>")
 endif()
 
 target_compile_options(Subprocess PRIVATE