[SCons] Disable Fortran for Apple clang by default

On Apple Silicon, the combination of gfortran with Apple clang compilers
causes hard crashes for F90 examples. Changing the default avoids this
issue; users can override by passing 'f90_interface=y' to SCons.

SConstruct

@@ -1411,6 +1411,9 @@ end program main
 
 set_fortran("{}FLAGS", env["FORTRANFLAGS"])
 
+if env["using_apple_clang"] and env["f90_interface"] == "default":
+    env["f90_interface"] = "n"
+
 if env['f90_interface'] in ('y','default'):
     foundF90 = False
     if env['FORTRAN']: