Provide a means to activate dynamically the debug traces

[Akim Demaille]

Hi all,

It is sometimes quite a pain in the neck to understand loading  
failures, especially when they happen on user side and we don't have  
access to their environment.  The following patch attempts to address  
this issue by giving a means to get debugging information.  It should  
certainly be refined with a means to select the verbosity level (or  
maybe some bitfield).

It helped me track something which I don't understand: when I dlopen a  
module.la which has a dependency.la library, I expected ltdl to  
automatically find dependency.la which is not installed.  Yet it does  
not seem to use the module.la's deplibs to dlopen them, it delegates  
this to the native dlopen.  As a result, since my library is not  
installed, dlopen fails (see dlopen-failure.txt attachement, notice  
the deplibs line).

Of course I can adjust LD_LIBRARY_PATH, but that's not what I expected.

Cheers!

dlopen-failure.txt
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0001-Dynamical-debug-traces.txt
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libtool.config
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