avoid depending on the libpcre version

This makes compiling pcre regex's lazier, so that they can don't depend upon the runtime version of libpcre. This would have helped with various issues such as #1221 and https://groups.google.com/d/msg/julia-users/HwXBv9v1mM4/o_W2NBXd1rQJ. It also makes #324 easy to implement.