tests/struct_flock.c: fix musl libc compilation warnings

tests/struct_flock.c: fix musl libc compilation warnings

The size of off_t is not something one can rely upon.  For example,
musl libc unconditionally defines it as an int64_t type on x86.
A cast to the target type helps to avoid these libc differences.

* configure.ac: Call AC_C_TYPEOF.
* tests/struct_flock.c (TYPEOF_FLOCK_OFF_T): New macro.
(test_flock_einval): Use it instead of off_t.