ExternalProject: fix the build command for generator overrides

The `binary_dir` variable is never set, so this is invalid. Instead, use
"." which all the other build commands use anyways. Also only set the
--config option if it is meaningful.

Modules/ExternalProject.cmake

@@ -1197,7 +1197,10 @@ function(_ep_get_build_command name step cmd_var)
         else()
           set(cmd "${CMAKE_COMMAND}")
         endif()
-        set(args --build ${binary_dir} --config ${CMAKE_CFG_INTDIR})
+        set(args --build ".")
+        if (CMAKE_CFG_INTDIR AND NOT CMAKE_CFG_INTDIR STREQUAL ".")
+          list(APPEND args --config "${CMAKE_CFG_INTDIR}")
+        endif ()
         if(step STREQUAL "INSTALL")
           list(APPEND args --target install)
         endif()