Revert "Remove 'compare_strings()' in favor of strcmp()"

This reverts commit 002b8f486d4e7c718eb6e4f565fddeb7f24b522d.

Because same-second checkout vs make rebuilding things
can sometimes screw you over when running tests, and strcmp()
doesn't handle NULL pointers, which the somewhat retarded
compare_strings() function does.

Signed-off-by: Andreas Ericsson <ae@op5.se>