perl-net-ssleay: fix build

set the correct prefix, instead of '/usr' detected on the host

see http://autobuild.buildroot.net/results/135/135867ef85535863e3647cc5fb82accb6f77612c/
perl-net-ssleay: fix build

the following error occurs after the previous patch
    arm-none-linux-gnueabi-gcc: error: unrecognized command line option '-shared '

so, force LDDLFLAGS without quote

note: previously LDDLFLAGS was correct