Merge bitcoin#32491: build: document why we check for std::system

8f4ba90 build: document why we check for std::system (fanquake)

Pull request description:

  It's probably debatable if we support targets like iOS, but for now, document why we are checking for this standard library feature.

  Trying to use `std::system` for a `aarch64-darwin-ios` target results in:
  ```bash
  test.cpp:7:10: error: 'system' is unavailable: not available on iOS
      7 |     std::system("some_command");
        |          ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_stdlib.h:203:6: note: 'system' has been explicitly marked unavailable here
    203 | int      system(const char *) __DARWIN_ALIAS_C(system);
        |          ^
  1 error generated.
  ```

ACKs for top commit:
  Sjors:
    ACK 8f4ba90

Tree-SHA512: 219cac205b36004c607194f6956c2ce6153f192bd4349e505b52c4e511e403e195ce0f462ae10c515e67f1e95d4b1636d526c8e4376004044853b574a84df427

Author: fanquake

cmake/introspection.cmake

@@ -19,6 +19,8 @@ endif()
 include(TestAppendRequiredLibraries)
 test_append_atomic_library(core_interface)
 
+# Even though ::system is part of the standard library, we still check
+# for it, to support building targets that don't have it, such as iOS.
 check_cxx_symbol_exists(std::system "cstdlib" HAVE_STD_SYSTEM)
 check_cxx_symbol_exists(::_wsystem "stdlib.h" HAVE__WSYSTEM)
 if(HAVE_STD_SYSTEM OR HAVE__WSYSTEM)