CMakeAddFortranSubdirectory: Simplfy logic to always build subdir

In the case that we use ExternalProject_Add to drive the subdirectory
build, replace the `forcebuild` step with the official `BUILD_ALWAYS`
option.

Issue: #20179

Modules/CMakeAddFortranSubdirectory.cmake

@@ -149,17 +149,9 @@ function(cmake_add_fortran_subdirectory subdir)
     -P ${build_dir}/config_mingw.cmake
     BUILD_COMMAND ${CMAKE_COMMAND}
     -P ${build_dir}/build_mingw.cmake
+    BUILD_ALWAYS 1
     INSTALL_COMMAND ""
     )
-  # make the external project always run make with each build
-  externalproject_add_step(${project_name}_build forcebuild
-    COMMAND ${CMAKE_COMMAND}
-    -E rm -f
-    ${CMAKE_CURRENT_BUILD_DIR}/${project_name}-prefix/src/${project_name}-stamp/${project_name}-build
-    DEPENDEES configure
-    DEPENDERS build
-    ALWAYS 1
-    )
   # create imported targets for all libraries
   foreach(lib ${libraries})
     add_library(${lib} SHARED IMPORTED GLOBAL)