respect multilib structure

[Alessandro-Barbieri]

currently these two files

   usr/lib/builtin.hexplug                                                                                            
   usr/lib/libimhex.so

get installed in lib while they must be in lib64 for 64 bit architectures

[iTrooz]

/usr/lib64 seems to be a good choice, but I don't see why /usr/lib is a bad choice

According to this post https://unix.stackexchange.com/questions/74646/difference-between-lib-lib32-lib64-libx32-and-libexec /usr/lib should contains "Libraries for programming and packages.", and I don't see any particular mention of architecture. If every library was put in lib64/lib32/etc... according to it's architecture, wouldn't there be nothing in /usr/lib then ?

Plus, AFAIK CMake generates the directory structure like this, and.. Shouldn't CMake's defaults be the right ones ?

@Alessandro-Barbieri Could you elaborate on this issue please ?