Define RN_BUILDING for React Native's own CocoaPods targets

Summary:
React Native's public C++ headers are gaining guards from `react/cxxstableapi`. Those
guards turn a direct include of a fine-grained header into an error for consumers that
opt into the strict API by defining `RN_STRICT_API`. React Native's own sources keep
including those fine-grained headers directly, so they have to be exempt. The guards
recognise the exemption via `RN_BUILDING`.

Mark every first-party pod as part of React Native's own build by defining
`RN_BUILDING` for it, through a new `mark_as_react_native_build` helper called last in
each spec block

Third-party pods deliberately do NOT get this: they compile their own code against the
public API and must remain subject to the guards. For the same reason the definition
goes into `pod_target_xcconfig`, which applies to the pod's own target, rather than
`user_target_xcconfig`, which would leak it to the app.

This change is inert on its own: nothing behaves differently unless a consumer defines
`RN_STRICT_API`.

Changelog: [Internal]

Differential Revision: D115051089

Author: coado

packages/react-native/Libraries/ActionSheetIOS/React-RCTActionSheet.podspec

@@ -31,4 +31,6 @@ Pod::Spec.new do |s|
   s.header_dir             = "RCTActionSheet"
 
   s.dependency "React-Core/RCTActionSheetHeaders", version
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/AppDelegate/React-RCTAppDelegate.podspec

@@ -86,4 +86,6 @@ Pod::Spec.new do |s|
   depend_on_js_engine(s)
   add_rn_third_party_dependencies(s)
   add_rncore_dependency(s)
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/Blob/React-RCTBlob.podspec

@@ -56,4 +56,6 @@ Pod::Spec.new do |s|
 
   add_rn_third_party_dependencies(s)
   add_rncore_dependency(s)
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/FBLazyVector/FBLazyVector.podspec

@@ -28,4 +28,5 @@ Pod::Spec.new do |s|
   s.source_files           = podspec_sources("**/*.{c,h,m,mm,cpp}", "**/*.h")
   s.header_dir             = "FBLazyVector"
 
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/Image/React-RCTImage.podspec

@@ -54,4 +54,6 @@ Pod::Spec.new do |s|
 
   add_rn_third_party_dependencies(s)
   add_rncore_dependency(s)
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/LinkingIOS/React-RCTLinking.podspec

@@ -48,4 +48,6 @@ Pod::Spec.new do |s|
   add_dependency(s, "React-RCTFBReactNativeSpec")
   add_dependency(s, "ReactCommon", :subspec => "turbomodule/core", :additional_framework_paths => ["react/nativemodule/core"])
   add_dependency(s, "React-NativeModulesApple", :additional_framework_paths => ["build/generated/ios"])
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/NativeAnimation/React-RCTAnimation.podspec

@@ -52,4 +52,6 @@ Pod::Spec.new do |s|
 
   add_rn_third_party_dependencies(s)
   add_rncore_dependency(s)
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/Network/React-RCTNetwork.podspec

@@ -54,4 +54,6 @@ Pod::Spec.new do |s|
 
   add_rn_third_party_dependencies(s)
   add_rncore_dependency(s)
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/PushNotificationIOS/React-RCTPushNotification.podspec

@@ -50,4 +50,6 @@ Pod::Spec.new do |s|
   add_dependency(s, "React-NativeModulesApple")
 
   add_rncore_dependency(s)
+
+  mark_as_react_native_build(s)
 end

packages/react-native/Libraries/Required/RCTRequired.podspec

@@ -27,4 +27,6 @@ Pod::Spec.new do |s|
   s.source                 = source
   s.source_files           = podspec_sources("**/*.{c,h,m,mm,cpp}", "**/*.h")
   s.header_dir             = "RCTRequired"
+
+  mark_as_react_native_build(s)
 end