CheckSymbolExists: Restore support for CMAKE_REQUIRED_FLAGS

Refactoring in commit a72e241200 (Check*: Restore more toleration of
`;-W...` in CMAKE_REQUIRED_FLAGS, 2026-07-21, v4.4.1~12^2) changed the
pattern that `Check*` modules use to forward `CMAKE_REQUIRED_FLAGS` to
`try_compile` calls.  However, it accidentally left out part of the new
pattern in the `CheckSymbolExists` module.  Fix that and add tests.

Fixes: #28001

Modules/CheckSymbolExists.cmake

@@ -174,6 +174,8 @@ int main(int argc, char** argv)
       ${_CSE_ADD_LINK_OPTIONS}
       ${_CSE_ADD_LINK_LIBRARIES}
       CMAKE_FLAGS
+        -DCOMPILE_DEFINITIONS:STRING=${CMAKE_REQUIRED_FLAGS}
+        ${_CSE_EXTRA_CMAKE_ARGUMENTS}
         -DCMAKE_SKIP_RPATH:BOOL=${CMAKE_SKIP_RPATH}
         "${_CSE_INCLUDE_DIRECTORIES}"
         "${_CSE_LINK_DIRECTORIES}"

Tests/CMakeOnly/CheckCXXSymbolExists/CMakeLists.txt

@@ -14,6 +14,13 @@ set(CMAKE_REQUIRED_INCLUDES "${CMAKE_CURRENT_SOURCE_DIR}/../CheckSymbolExists")
 
 include(CheckCXXSymbolExists)
 
+set(CMAKE_REQUIRED_FLAGS "-DCSE_REQUIRED_FLAG")
+check_cxx_symbol_exists(required_flag_symbol "cm_cse.h" CSE_RESULT_REQUIRED_FLAG)
+if(NOT CSE_RESULT_REQUIRED_FLAG)
+  message(SEND_ERROR "CheckCXXSymbolExists did not honor CMAKE_REQUIRED_FLAGS")
+endif()
+unset(CMAKE_REQUIRED_FLAGS)
+
 foreach(_config_type Release RelWithDebInfo MinSizeRel Debug)
   set(CMAKE_TRY_COMPILE_CONFIGURATION ${_config_type})
   unset(CSE_RESULT_${_config_type} CACHE)

Tests/CMakeOnly/CheckSymbolExists/CMakeLists.txt

@@ -14,6 +14,13 @@ set(CMAKE_REQUIRED_INCLUDES "${CMAKE_CURRENT_SOURCE_DIR}")
 
 include(CheckSymbolExists)
 
+set(CMAKE_REQUIRED_FLAGS "-DCSE_REQUIRED_FLAG")
+check_symbol_exists(required_flag_symbol "cm_cse.h" CSE_RESULT_REQUIRED_FLAG)
+if(NOT CSE_RESULT_REQUIRED_FLAG)
+  message(SEND_ERROR "CheckSymbolExists did not honor CMAKE_REQUIRED_FLAGS")
+endif()
+unset(CMAKE_REQUIRED_FLAGS)
+
 foreach(_config_type Release RelWithDebInfo MinSizeRel Debug)
   set(CMAKE_TRY_COMPILE_CONFIGURATION ${_config_type})
   unset(CSE_RESULT_${_config_type} CACHE)

Tests/CMakeOnly/CheckSymbolExists/cm_cse.h

@@ -3,4 +3,8 @@
 
 int non_existent_function_for_symbol_test();
 
+#ifdef CSE_REQUIRED_FLAG
+#  define required_flag_symbol 1
+#endif
+
 #endif