Issue with own implementation of strcasecmp

[michal-josef-spacek]

This goes from #1203.
The libredwg own implementation of strcasecmp() is bad.

The proposal:

1. Write tests.
2. Fix the implementation.

When I wrote tests, I found question how to test it. My intention is possibility of test on every system. When I skip system strcasecmp via ./configure ac_cv_func_strcasecmp=no, problem goes to error with library, where is still strcasecmp.

From this I have another proposal:
Rename strcasecmp to something own_strcasecmp and redefine it to strcasecmp in case of #ifndef HAVE_STRCASECMP.

[michal-josef-spacek]

@rurban What do you think?

[rurban]

It should be named my_strcasecmp. I wrote one some time ago.

But it's a one line fix only here