mail/mailagent: clean up fallout to fix build...

patch parser.c's check_fatal to avoid the int-conversion errors,
and quench some warnings. Untested on FreeBSD 14, but
tested on 13.2 with clang15.

https://pkg-status.freebsd.org/beefy18/data/main-amd64-default/p54373d87b552_s4194bbb34c/logs/mailagent-3.1.106.log

Note the code uses K&R style function declarations, not proper
prototypes, which upset newer versions of the clang compiler.
So pretend this were C99 code, too.

While here, propagate CC to the build.