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The find_library_function() in Lib/distutils/unixccompiler.py does a very simple-minded check to determine the existence of a library. It basically only checks that a certain .so file exists. This may lead to false positives: the mere existence of a .so file does not imply that we can actually link against that library.
In addition to (or even better: instead of) checking the existence of the file, you should try to (in the spirit of autoconf) compile a simple program using $CC -l$LIB prog.c -o prog
One particular instance of where things can go wrong is with a 32-bit/64-bit multilib installation. Python might find a 64-bit library in /usr/lib when we're actually compiling 32-bit with libraries in /usr/lib32.