[Build] Disable Concurrency if the deployment version is too low.

For the new runtime build only (and only when triggered from the old
build system), turn Concurrency off if the deployment target is less
than 10.15.

CMakeLists.txt

@@ -1640,6 +1640,12 @@ if(SWIFT_ENABLE_NEW_RUNTIME_BUILD)
         set(stdlib_deployment_version_flag -DCMAKE_OSX_DEPLOYMENT_TARGET=${SWIFT_SDK_${sdk}_DEPLOYMENT_VERSION})
       endif()
 
+      if(sdk STREQUAL "OSX" AND SWIFT_SDK_${sdk}_DEPLOYMENT_VERSION VERSION_LESS "10.15")
+        set(build_concurrency NO)
+      else()
+        set(build_concurrency YES)
+      endif()
+
       ExternalProject_Add("${stdlib_target}-core"
         SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/Runtimes/Core"
         # TODO: Add this once we're ready to start swapping out the libraries
@@ -1665,7 +1671,7 @@ if(SWIFT_ENABLE_NEW_RUNTIME_BUILD)
           -DCMAKE_COLOR_DIAGNOSTICS:BOOLEAN=${CMAKE_COLOR_DIAGNOSTICS}
           -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
           -DSwiftCore_INSTALL_NESTED_SUBDIR=YES
-          -DSwiftCore_ENABLE_CONCURRENCY=YES)
+          -DSwiftCore_ENABLE_CONCURRENCY=${build_concurrency})
       if(NOT ${CMAKE_CROSSCOMPILING})
         add_dependencies("${stdlib_target}-core" swift-frontend)
       endif()