For some reason -g is not ending up in CFLAGS for some builds in Travis, 
which is likely a configure script bug of some kind.

Work around it by defaulting CFLAGS to -g which should be always
harmless.
e.g. https://travis-ci.com/github/apache/httpd/jobs/526267899

Github: closes #205


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1891806 13f79535-47bb-0310-9956-ffa450edef68
