If --enable-suexec is specified, then we need to build suexec

If --enable-suexec is specified, then we need to build suexec
automatically.  This bit me multiple times last night as I was trying to
setup apache.org.  I would make a change to the configuration, run make;
make install, and it would fail.  The problem was that suexec wasn't
getting re-compiled ever, so my changes weren't getting picked up.  This
solves that problem, by making sure that suexec gets compiled if it was
specified on the configure line.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87779 13f79535-47bb-0310-9956-ffa450edef68