Custom location for postgis extension .control file
This is not 100% proven, but the following pass of the documentation suggests you just cannot have your control files in other locations:


  extension_name
  
  The name of the extension to be installed. PostgreSQL will create the extension using details from the file SHAREDIR/extension/extension_name.control.


I can imagine one can work it around on systems supporting symbolic links - but in your case this does not help as well, as you cannot create the symlink at the right location either.