Fix linking on Gentoo

For the lib32 paths: If you pass -m32 in {C,CXX}FLAGS environment variables or pass it in -DCMAKE_{C,CXX}_FLAGS on the *command-line*, CMake would automatically detect a 32-bit build and disables searching in lib64 paths. The same is accomplished by setting the FIND_LIBRARY_USE_LIB64_PATHS
global property.

However, it still doesn't search in lib32 variants, only the generic lib directories. As lib contains 64-bit libraries on some distributions this will fail.

Curiously, CMake *sometimes* magically transforms /usr/lib/libfoobar.so paths into -lfoobar before generating the makefile, which will allow the linker to find the correct library. This didn't happen here for me.

There is a [CMake bug report with a patch for this](http://public.kitware.com/Bug/view.php?id=11260) but it seems to have been abandoned.