Define RN_BUILDING for React Native's own CocoaPods targets (#57858)

Summary:
Pull Request resolved: #57858

`RN_BUILDING` marks React Native's own targets so the react/cxxstableapi guards stay inert for internal sources, which keep including fine-grained headers directly.

It is required whenever `RN_STRICT_API` reaches React Native's own compilation rather than only the consumer's:
- Project-wide enablement — a consumer applying the flag to every pod target (the CocoaPods post_install idiom) or through a global Buck config. Without RN_BUILDING, React Native's own translation units fail against their own guards.
- Private headers — `PrivateGuard.h` has no umbrella escape (#if defined(RN_STRICT_API) && !defined(RN_BUILDING)), so RN_BUILDING is the only way internal code can include them at all.

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

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

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D115051089

fbshipit-source-id: f1178e2d0727827e5f40fc7b4a8f55b4d9b505cc

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

@@ -87,4 +87,6 @@ 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