FortranCInterface: Fix regression in timestamp check

Since commit d7c8030541 (FortranCInterface: Fix misuse of IS_NEWER_THAN
in timestamp check, 2021-02-21, v3.21.0-rc1~631^2~3), FortranCInterface
checks for `Output.cmake.in` in the build tree instead of the source
tree as before.  This caused it to always re-run the detection.

Fixes: #22709

Modules/FortranCInterface/Detect.cmake

@@ -8,11 +8,10 @@ configure_file(${FortranCInterface_SOURCE_DIR}/Input.cmake.in
 # configuration changes.
 if(NOT EXISTS ${FortranCInterface_BINARY_DIR}/Output.cmake
     OR NOT EXISTS ${FortranCInterface_BINARY_DIR}/Input.cmake
-    OR NOT EXISTS ${FortranCInterface_BINARY_DIR}/Output.cmake.in
     OR NOT ${FortranCInterface_BINARY_DIR}/Output.cmake
       IS_NEWER_THAN ${FortranCInterface_BINARY_DIR}/Input.cmake
     OR NOT ${FortranCInterface_SOURCE_DIR}/Output.cmake
-      IS_NEWER_THAN ${FortranCInterface_BINARY_DIR}/Output.cmake.in
+      IS_NEWER_THAN ${FortranCInterface_SOURCE_DIR}/Output.cmake.in
     OR NOT ${FortranCInterface_BINARY_DIR}/Output.cmake
       IS_NEWER_THAN ${FortranCInterface_SOURCE_DIR}/CMakeLists.txt
     OR NOT ${FortranCInterface_BINARY_DIR}/Output.cmake