Improve check for F18 error stop

- ifort 21.1 BETA 20200827 compiles F18 error stop syntax but
  does not propagate the error code correctly

Author: awvwgk

CMakeLists.txt

@@ -35,7 +35,7 @@ endif()
 # --- compiler feature checks
 include(CheckFortranSourceCompiles)
 include(CheckFortranSourceRuns)
-check_fortran_source_compiles("error stop i; end" f18errorstop SRC_EXT f90)
+check_fortran_source_runs("i=0; error stop i; end" f18errorstop SRC_EXT f90)
 check_fortran_source_compiles("real, allocatable :: array(:, :, :, :, :, :, :, :, :, :); end" f03rank SRC_EXT f90)
 check_fortran_source_runs("use, intrinsic :: iso_fortran_env, only : real128; real(real128) :: x; x = x+1; end" f03real128)