[Vortex] Impossible to configure location of DTD files

[Jens Alfke]

I've run into a problem using Vortex on Mac OS X. I've built the  
Vortex library as a "framework", which is the preferred way to package  
up libraries with their headers and resource files. However,  
vortex_init looks for the DTD files only in the current directory  
(which will not be the framework's directory), and it's illegal to to  
add the framework's resource directory by calling  
vortex_support_add_search_path before calling vortex_init.

Looking at the source code, the way Vortex deals with this on other  
platforms is to use a hardcoded path set in the preprocessor variable  
PACKAGE_DTD_DIR. Unfortunately, frameworks can be installed in  
multiple places, or embedded inside apps, so the path isn't fixed — it  
needs to be looked up at runtime.

One way to fix this would be to add some #ifdef'ed Mac-specific code  
to vortex_init. But that seems like a kludge, and besides, someone  
else might want to package Vortex differently on Mac OS, so that  
configuration wouldn't be right for them.

A better solution, I think, would be to wait to load the DTD files  
until they're needed. That way the call to vortex_init will succeed,  
and the client can then vortex_support_add_search_path with the  
appropriate path before opening a connection. I'm not sure I  
understand the library well enough to make such a change, though...

--Jens