On modern systems (e.g., FreeBSD), the db* functions reside in libc

rather than in libdb. Also, be more precise in telling what is actually
checked for (we are NOT testing for main(), but we are testing for dbopen(),
right?).
Without the first fix, configuration would fail completely for FreeBSD


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90391 13f79535-47bb-0310-9956-ffa450edef68