[CMake] new rpath support

[Brad King]

Zachary Pincus wrote:
> Here's what I'm trying to do in a cross-platform way, hopefully with  
> cmake. I am creating several shared libraries (python modules,  
> actually), which share some runtime code. Thus, each module needs to  
> link to a secondary shared object that contains this runtime.
> 
> What are my options for making sure that when one of these python  
> modules is loaded, the dynamic loader will be able to locate the  shared 
> runtime module? I see the following, of which (3) seems  optimal, except 
> that I'm not sure I can do that on windows.
> 
> The options:
> (1) Set with PATH (win) or LD_LIBRARY_PATH (unix) environment  variable 
> to tell the loader where to look. This has to be done before  the 
> program starts, so the onus is on the user to make sure these are  set 
> right. That's a pain.

...but it is the accepted way for users to deal with things they install 
to their home directories without admin priviledges.

> (2) Install the library to a standard location where the loader knows  
> to look. This is /usr/lib or /usr/local/lib on unix. I'm not sure how  
> to do this in windows -- does the dll go somewhere and the import  
> library go somewhere else? Not sure. Anyhow, globally installing  
> something seems nasty (and requires admin privileges).

This will work for those with admin priviledges and for system-provided 
packages.

> (3) Install the library in a location with a known relative path to  the 
> python modules. On OS X, link the *runtime library* with an  
> install_name containing this relative path. On other unix, link each  
> *python module* with an rpath containing that relative path. (Is this  
> right??) On windows... is this even possible?

This may work on OSX if the install_name uses the @executable_path 
feature but it will not work in general.  RPATH entries cannot be 
relative paths because they would be interpreted relative to the current 
working directory, not the executable or module location!

The solution on Windows is just to put the runtime DLLs in the same 
directory as the modules that need them.

-Brad