Concat string at compile time when possible.

Doing so, sometimes also give the opportunity to turn a 'ap_fputstrs' into a 'ap_fputs'.
PR 53741

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1626050 13f79535-47bb-0310-9956-ffa450edef68