Two more mod_so configure-time fixes:

 - Treat --enable-so=yes as "static" (this includes --enable-so, etc).
 - An explicit --enable-so=shared issues an error.

Put in a note about the last condition that I'm not as sure how to fix:
If the user doesn't explicitly request mod_so, but instead gives a
mass-enable parameter like --enable-modules=most or
--enable-mods-shared=most then it is still possible to enable a bunch
of modules while mod_so itself is not buildable (ie no APR_HAS_DSO or
other build dependency failure).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91658 13f79535-47bb-0310-9956-ffa450edef68