Further complication with the new email@example.com_0: on El Capitan, exiv2 succeeds (the missing symbol being defined there in /usr/include/pthread/pthread.h, which doesn't exist on Snow Leopard), but libextractor fails with the new exiv2, due to the following (which isn't a problem with the older firstname.lastname@example.org_0):
:info:build exiv2_extractor.cc:183:20: error: virtual function 'size' has a different return type ('long') than the function it overrides (which has return type 'size_t' (aka 'unsigned long'))
:info:build virtual long int size (void) const;
:info:build ~~~~ ^
:info:build /opt/local/include/exiv2/basicio.hpp:225:24: note: overridden virtual function is here
:info:build virtual size_t size() const = 0;
:info:build ~~~~~~ ^
:info:build 1 error generated.
Fortunately, nothing I have at the moment actually needs libextractor, so I could just uninstall it.