Make fmtstring in datetimeToPosixString flexible size.

src/libmtime.f90

@@ -1048,7 +1048,7 @@ contains
   subroutine datetimeToPosixString(my_datetime, string, fmtstr, errno) !OK-TESTED.
     type(datetime), pointer :: my_datetime
     character(len=max_datetime_str_len) :: string
-    character(len=32) :: fmtstr
+    character(len=*) :: fmtstr
     type(c_ptr) :: dummy_ptr
     integer :: i
     integer, optional:: errno