diff --git a/cmake/introspection.cmake b/cmake/introspection.cmake
index e8538ae73cd..0efcf7c63c7 100644
--- a/cmake/introspection.cmake
+++ b/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)