Fix building without perl in the build root
    
    After building extensions, Makefile builds documentation by executing
    the new perl. Because we version libperl.so soname, we need to
    precreate a symlink from the soname to ./libperl.so so that the new
    perl can be executed. Otherwise this happens:
    
    LD_LIBRARY_PATH=/builddir/build/BUILD/perl-5.24.0  ./perl -Ilib -f pod/buildtoc -q
    ./perl: error while loading shared libraries: libperl.so.5.24: cannot open shared object file: No such file or directory
    
    This did not appear on upgrade from previous perl version because perl
    build script sets LD_PRELOAD in addition if libperl.so exists in the system.