Wrap most of the defines in an ifndef to allow them to be redefined

elsewhere (eg. EXTRA_CFLAGS) without having to edit httpd.h.  This makes
vendor distributions of Apache easier to maintain.

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Submitted by:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@78882 13f79535-47bb-0310-9956-ffa450edef68