Two things have been changed in upstream: build system now properly
places 64-bit .so under %{_libdir}, and the .so itself now contains
versioned SONAME.

This means we can remove workaround for the first issue and start
packaging library separately.  This is much closer to FPG than before.

Ref.: https://github.com/naelstrof/slop/issues/86#issuecomment-328479278

It was moved to slop subdirectoty in attempt to get rid of rpmlint
warning concerning non-standard soname:  this would make 'libslopy.so' a
"private" library, for which naming is not mandatory.

However, this actually breaks slop binary!

Turns out there's no way around it.  What we could do is, in order of
preference:

 1. Solve naming issue with upstream.

 2. Just call it libslopy.so.1.

 3. Use wrapper to add %{_libdir}/slop to LD_LIBRARY_PATH

(The last thing seems really like a too hacky for such simple utility.)

Since libslopy is considered private, there should not be need for
header file.