Revert of Mojo: Eliminate duplicate C API symbols

mojo/edk/system/BUILD.gn

Index: mojo/edk/system/BUILD.gn
diff --git a/mojo/edk/system/BUILD.gn b/mojo/edk/system/BUILD.gn
index aaf3aabae282ac05cca5c9351c42a06f00f0a1f4..dd153d6b1bc1a7253c58528edb558f3ed49263c8 100644
--- a/mojo/edk/system/BUILD.gn
+++ b/mojo/edk/system/BUILD.gn
@@ -9,6 +9,13 @@
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
+}
+
+config("system_config") {
+  defines = [
+    # Ensures that dependent projects import the core functions on Windows.
+    "MOJO_USE_SYSTEM_IMPL",
+  ]
 }
 
 component("system") {
@@ -75,7 +82,12 @@
     "watcher_set.h",
   ]
 
-  defines = [ "MOJO_SYSTEM_IMPL_IMPLEMENTATION" ]
+  defines = [
+    "MOJO_SYSTEM_IMPL_IMPLEMENTATION",
+    "MOJO_SYSTEM_IMPLEMENTATION",
+  ]
+
+  all_dependent_configs = [ ":system_config" ]
 
   public_deps = [
     "//mojo/edk/embedder",