Apple: Restore detection of platform-specific GNU Fortran flags

Detection of `-isysroot` was broken by commit a1b5c0da87 (Modules:
Tolerate CMAKE_EXECUTE_PROCESS_COMMAND_ERROR_IS_FATAL, 2026-05-05,
v4.4.0-rc1~209^2~1) because `gfortran -v --help` returns non-zero.

Fixes: #28111

Modules/Platform/Apple-GNU.cmake

@@ -31,9 +31,9 @@ macro(cmake_gnu_set_sysroot_flag lang)
       COMMAND ${CMAKE_${lang}_COMPILER} "-v" "--help"
       OUTPUT_VARIABLE _gcc_help
       ERROR_VARIABLE _gcc_help
-      RESULT_VARIABLE _gcc_result
+      RESULT_VARIABLE _gcc_result # may be non-zero
       )
-    if(_gcc_result EQUAL 0 AND "${_gcc_help}" MATCHES "isysroot")
+    if("${_gcc_help}" MATCHES "isysroot")
       message(CHECK_PASS "yes")
       set(CMAKE_${lang}_SYSROOT_FLAG "-isysroot")
     else()
@@ -52,9 +52,9 @@ macro(cmake_gnu_set_osx_deployment_target_flag lang)
       COMMAND ${CMAKE_${lang}_COMPILER} "-v" "--help"
       OUTPUT_VARIABLE _gcc_help
       ERROR_VARIABLE _gcc_help
-      RESULT_VARIABLE _gcc_result
+      RESULT_VARIABLE _gcc_result # may be non-zero
       )
-    if(_gcc_result EQUAL 0 AND "${_gcc_help}" MATCHES "macosx-version-min")
+    if("${_gcc_help}" MATCHES "macosx-version-min")
       message(CHECK_PASS "yes")
       set(CMAKE_${lang}_OSX_DEPLOYMENT_TARGET_FLAG "-mmacosx-version-min=")
     else()