Define RN_BUILDING for React Native's own CMake, SwiftPM, and Buck targets

Summary:
Companion to the CocoaPods change: does the same for the other three build systems, so
every build has exactly one place that states "these targets are React Native's own".

React Native's public C++ headers are gaining guards from `react/cxxstableapi`, which
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 headers directly, so they are exempted via `RN_BUILDING`.

Unlike CocoaPods, these three build systems each have a single chokepoint:

- CMake: one `add_compile_definitions(RN_BUILDING)` in the ReactAndroid JNI project,
  a directory property inherited by every `add_react_common_subdir` below it. It is
  declared *after* the third-party NDK subdirectories so glog/boost/folly/fmt never see
  it, and this project never compiles app or third-party module code — those build
  against the prefab artifacts through `ReactNative-application.cmake`. For that reason
  the flag deliberately does NOT go into `target_compile_reactnative_options`, which app
  and third-party module builds do call.
- SwiftPM: one `.define` in the shared `Target.reactNativeTarget` factory that every
  React Native target is created through. `cxxSettings` are per-target and are not
  inherited by packages that depend on React.
- Buck: a `_set_rn_building_flag` helper called from the four macros React Native's own
  targets use. These macros are also used by ~300 product packages that *consume* React
  Native, and those must stay subject to the guards, so the flag is scoped by package
  prefix rather than applied unconditionally. It is `preprocessor_flags`, deliberately
  not `exported_preprocessor_flags`, so dependents are not exempted either.

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

Changelog: [Internal]

Differential Revision: D115051088

Author: Dawid Małecki

packages/react-native/Package.swift

@@ -948,6 +948,10 @@ extension Target {
         CXXSetting.headerSearchPath(relativeSearchPath(numOfSlash + 1, ".build/headers/React")),
       ]
 
+    // Every target built through this factory is React Native's own, so RN_BUILDING
+    // keeps the react/cxxstableapi guards inert for internal sources. cxxSettings are
+    // per-target and are not inherited by packages that depend on React, so this does
+    // not exempt consumers from the guards.
     let cxxSettings =
       [
         .unsafeFlags(["-std=c++20"]),
@@ -956,6 +960,7 @@ extension Target {
         .define("USE_HERMES", to: "1"),
         .define("RCT_REMOVE_LEGACY_ARCH", to: "1"),
         .define("HERMES_V1_ENABLED", to: "1"),
+        .define("RN_BUILDING", to: "1"),
       ] + defines + cxxCommonHeaderPaths
 
     return .target(

packages/react-native/ReactAndroid/src/main/jni/CMakeLists.txt

@@ -58,6 +58,14 @@ add_react_third_party_ndk_subdir(fast_float)
 add_react_third_party_ndk_subdir(fmt)
 add_react_third_party_ndk_subdir(folly)
 
+# Marks everything added below as React Native's own build, so the
+# react/cxxstableapi guards let internal sources keep including the
+# fine-grained headers they fence off from consumers. Deliberately declared
+# after the third-party subdirectories so it never reaches them, and this
+# project never compiles app or third-party module code (those build against
+# the prefab artifacts through ReactNative-application.cmake).
+add_compile_definitions(RN_BUILDING)
+
 # Common targets
 add_react_common_subdir(yoga)
 add_react_common_subdir(runtimeexecutor)