use C11 _Noreturn instead of GNUC-attribute
changes in read_file in gbdasm.c

- data is now an argument, not returned
- appending a \0 to the end is unneccessary
- use of temporary variables to overwrite at the end
change order in which newlines and comments are skipped
change comment and add void-argument in scan_special_identifier
add recursive calls to Makefile in test directory
remove variables_init, as it was a bit nonsensical
reset global variables in variables_destroy
remove redundant extern definition and add comment for another
make this program conform to standards!