Fix: resolve build errors

[jimmale]

The initial motivation for this PR was to resolve compiler warnings, which are promoted to errors, which cause the build to fail. I ended up also fixing a couple issues with some of the tests (other issues still remain).

• (gcc) Strings in ais_charset.c, interactive.c, and uat_decode.c had character arrays initialized without a null terminator. GCC's proposed fix is not portable to clang, so I've removed the hard-coded array length. Both compilers seem to be satisfied with this. (See also, Fix: Increase spinner array size for GCC 14/15 compatibility #10)
• (clang) The value of res in geomag.c was not being used. I've added appropriate checks to use the value as intended.
• (clang) Certain variables in globe_index.c were being written, but never read. I've removed the lines of code in question.
• The CRC tests were failing due to the setExit symbol being missing, so I've added a stubbed version for the tests.
• Small changes to the Makefile for ergonomics

Tested on the following setup:

$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=25.10
DISTRIB_CODENAME=questing
DISTRIB_DESCRIPTION="Ubuntu 25.10"

$ gcc --version
gcc (Ubuntu 15.2.0-4ubuntu4) 15.2.0
Copyright (C) 2025 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ clang --version
Ubuntu clang version 20.1.8 (0ubuntu4)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-20/bin

Full disclosures:

• C is not my strongest language
• This PR does not contain AI-generated code