Also look in libc for pthread_create before deciding the system doesn't
have pthreads support. Not all systems have libpthread (eg, BSD/OS). Many compiler options that were not particular to whether threads were being used were being set in a threads-specific block, meaning they didn't get set at all if threads were not being used. ISC_PLATFORM_USETHREADS could end up still defined even if threading was later disabled, such as if pthread_create was not found.