Make AP_SERVER_ADD_STRING define-able from CFLAGS.
backport of r1481955.
Reviewed by: minfrin, humbedooh


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1486262 13f79535-47bb-0310-9956-ffa450edef68