The value of `$?' on entrance to signal handlers in shell scripts
cannot be relied upon, so set the exit code explicitly.
* cfg.mk (sc_always_defined_macros, sc_system_h_headers): Set
the exit code in signal handler explicitly to 128 + SIG<SIGNAL>.
* src/Makefile.am (sc_tight_scope): Likewise.
* tests/test-lib.sh: Likewise.