Tests/FortranModules: Simplify check for submodules support

Tests/CMakeLists.txt

@@ -3260,6 +3260,9 @@ if(BUILD_TESTING)
         # FIXME(lfortran): The compiler fails on the test's modules.
         AND NOT CMAKE_Fortran_COMPILER_ID STREQUAL "LFortran"
         )
+      if(DEFINED CMake_TEST_Fortran_SUBMODULES)
+        list(PREPEND CMake_TEST_FortranModules_BUILD_OPTIONS -DCMake_TEST_Fortran_SUBMODULES:BOOL=${CMake_TEST_Fortran_SUBMODULES})
+      endif()
       add_test(FortranModules ${CMAKE_CTEST_COMMAND}
         --build-and-test
         "${CMake_SOURCE_DIR}/Tests/FortranModules"
@@ -3268,7 +3271,6 @@ if(BUILD_TESTING)
         --build-project FortranModules
         --build-options
           -DCMake_TEST_NESTED_MAKE_PROGRAM:FILEPATH=${CMake_TEST_EXPLICIT_MAKE_PROGRAM}
-          -DCMake_TEST_Fortran_SUBMODULES:BOOL=${CMake_TEST_Fortran_SUBMODULES}
           ${CMake_TEST_FortranModules_BUILD_OPTIONS}
         )
       list(APPEND TEST_BUILD_DIRS "${CMake_BINARY_DIR}/Tests/FortranModules")

Tests/FortranModules/CMakeLists.txt

@@ -10,9 +10,7 @@ if(NOT DEFINED CMake_TEST_NESTED_MAKE_PROGRAM AND NOT CMAKE_GENERATOR MATCHES "V
   set(CMake_TEST_NESTED_MAKE_PROGRAM "${CMAKE_MAKE_PROGRAM}")
 endif()
 
-if("x${CMake_TEST_Fortran_SUBMODULES}" STREQUAL "x"
-    AND NOT CMAKE_VERSION VERSION_LESS 3.6.20160923 # for CheckFortranSourceCompiles SRC_EXT
-    )
+if(NOT DEFINED CMake_TEST_Fortran_SUBMODULES)
   include(CheckFortranSourceCompiles)
   CHECK_Fortran_SOURCE_COMPILES([[
 module parent