Merge pull request swiftlang#1723 from compnerd/destination

build: strip DESTDIR when performing subinstall

Author: compnerd

CMakeLists.txt

@@ -38,7 +38,9 @@ ExternalProject_Add(CoreFoundation
                       -DCF_DEPLOYMENT_SWIFT=YES
                       -DCF_ENABLE_LIBDISPATCH=${FOUNDATION_ENABLE_LIBDISPATCH}
                       -DCF_PATH_TO_LIBDISPATCH_SOURCE=${FOUNDATION_PATH_TO_LIBDISPATCH_SOURCE}
-                      -DCF_PATH_TO_LIBDISPATCH_BUILD=${FOUNDATION_PATH_TO_LIBDISPATCH_BUILD})
+                      -DCF_PATH_TO_LIBDISPATCH_BUILD=${FOUNDATION_PATH_TO_LIBDISPATCH_BUILD}
+                    INSTALL_COMMAND
+                      ${CMAKE_COMMAND} -E env --unset=DESTDIR ${CMAKE_COMMAND} --build . --target install)
 ExternalProject_Get_Property(CoreFoundation install_dir)
 
 set(swift_optimization_flags)