Improve ISO C Conformance and Remove Warnings: Part 1

It's a small improvement, though. Tested with `-std=gnu99 -pedantic`.

Remaining GNU extensions are in `include/picrin/gc.h`, line 13 (there is no portable way to make `enum pic_gc_mark` a type of bit-field) and `src/vm.c` (getting an address of `goto` labels whether the direct threaded VM is enabled or not).