tests: Pth needs to be installed as a standard APE header/library

mkmk -m -S -B -I. -D_POSIX_SOURCE -D_SUSV2_SOURCE -D_RESEARCH_SOURCE -D_REENTRANT_SOURCE -D_BSD_EXTENSION -D_NET_EXTENSION -D_LIMITS_EXTENSION -D_C99_SNPRINTF_EXTENSION -D_REGEXP_EXTENSION -DHAVE_SOCK_OPTS -x test_common.c > mkfile